diff --git a/src/main/kotlin/exercise1/task1/GradeCalculator.kt b/src/main/kotlin/exercise1/task1/GradeCalculator.kt
index df111ab..d4a8883 100644
--- a/src/main/kotlin/exercise1/task1/GradeCalculator.kt
+++ b/src/main/kotlin/exercise1/task1/GradeCalculator.kt
@@ -40,13 +40,22 @@ import java.util.Scanner
  */
 
 internal fun calculateGrade(score: Int): Int {
-    TODO("Implement me!!!")
+    return when {
+        score in 0..50 -> 5
+        score in 51..60 -> 6
+        score in 61..70 -> 7
+        score in 71..80 -> 8
+        score in 81..90 -> 9
+        score in 91..100 -> 10
+        else -> throw IllegalArgumentException("Score out of range 0-100")
+    }
 }
 
 fun main() {
     print("Enter student score: ")
     val scanner = Scanner(System.`in`)
     val score = scanner.nextInt()
-
-    calculateGrade(score)
+    val grade = calculateGrade(score)
+    val msg = if(grade != 5) "Student's grade is: $grade" else "Student didn't pass the exam"
+    println(msg)
 }
diff --git a/src/main/kotlin/exercise1/task2/PrintPyramid.kt b/src/main/kotlin/exercise1/task2/PrintPyramid.kt
index d9c89c9..f2bc9db 100644
--- a/src/main/kotlin/exercise1/task2/PrintPyramid.kt
+++ b/src/main/kotlin/exercise1/task2/PrintPyramid.kt
@@ -57,7 +57,14 @@ package org.jetbrains.exercise1.task2
  */
 
 internal fun printPyramid(level: Int) {
-    TODO("Implement me!!!")
+    require (level in 3..15) {
+        "Level is out of range"
+    }
+    for(i in 1..level) {
+        val spaces = " ".repeat(level - i)
+        val stars = "*".repeat(2 * i - 1)
+        println("$spaces$stars$spaces")
+    }
 }
 
 fun main() {
diff --git a/src/main/kotlin/exercise1/task3/ScrabbleWordScoreCalculator.kt b/src/main/kotlin/exercise1/task3/ScrabbleWordScoreCalculator.kt
index c0e883c..64a6774 100644
--- a/src/main/kotlin/exercise1/task3/ScrabbleWordScoreCalculator.kt
+++ b/src/main/kotlin/exercise1/task3/ScrabbleWordScoreCalculator.kt
@@ -31,8 +31,18 @@ package org.jetbrains.exercise1.task3
  *
  */
 
+val charMap = mapOf(
+    'A' to 1, 'E' to 1, 'I' to 1, 'O' to 1, 'U' to 1, 'L' to 1, 'N' to 1, 'R' to 1, 'S' to 1, 'T' to 1,
+    'D' to 2, 'G' to 2,
+    'B' to 3, 'C' to 3, 'M' to 3, 'P' to 3,
+    'F' to 4, 'H' to 4, 'V' to 4, 'W' to 4, 'Y' to 4,
+    'K' to 5,
+    'J' to 8, 'X' to 8,
+    'Q' to 10, 'Z' to 10,
+)
+
 internal fun calculateWordScrabbleScore(word: String): Int {
-    TODO("Implement me!!!")
+    return word.sumOf { charMap.getOrDefault(it.uppercaseChar(), 0) }
 }
 
 fun main() {
diff --git a/src/main/kotlin/exercise1/task4/PalindromeNumber.kt b/src/main/kotlin/exercise1/task4/PalindromeNumber.kt
index 04a692d..2827e72 100644
--- a/src/main/kotlin/exercise1/task4/PalindromeNumber.kt
+++ b/src/main/kotlin/exercise1/task4/PalindromeNumber.kt
@@ -28,11 +28,19 @@ package org.jetbrains.exercise1.task4
  */
 
 internal fun isPalindrome(x: Int): Boolean {
-    TODO("Implement me!!!")
+    require(x in -1000000..1000000) {
+        "Number must be in range -1000000..1000000"
+    }
+    return when {
+        x < 0 -> false
+        x.toString().length == 1 -> true
+        x.toString() == x.toString().reversed() -> true
+        else -> false
+    }
 }
 
 fun main() {
-    val number = 121
+    val number = 56465
     if (isPalindrome(number)) {
         println("Number $number is a palindrome.")
     } else {
diff --git a/src/main/kotlin/exercise1/task5/ReverseInteger.kt b/src/main/kotlin/exercise1/task5/ReverseInteger.kt
index 447b8a1..bb12455 100644
--- a/src/main/kotlin/exercise1/task5/ReverseInteger.kt
+++ b/src/main/kotlin/exercise1/task5/ReverseInteger.kt
@@ -1,5 +1,7 @@
 package org.jetbrains.exercise1.task5
 
+import kotlin.math.abs
+
 /**
  * Task 5: Given an integer x, return x with its digits reversed.
  *
@@ -27,10 +29,11 @@ package org.jetbrains.exercise1.task5
  */
 
 internal fun reverseInteger(x: Int): Int {
-    TODO("Implement me!!!")
+    require(x in -1000000..1000000) { "Number must be integer in range -1000000..1000000" }
+    val reversed = abs(x).toString().reversed()
+    return if (x < 0) ("-$reversed").toInt() else reversed.toInt()
 }
-
 fun main() {
-    val integer = -321
+    val integer = -78952
     println("Reverse integer of number $integer is ${reverseInteger(integer)}")
 }
diff --git a/src/main/kotlin/exercise2/task1/FindPairOfHighestSum.kt b/src/main/kotlin/exercise2/task1/FindPairOfHighestSum.kt
index ddf425b..590c9c8 100644
--- a/src/main/kotlin/exercise2/task1/FindPairOfHighestSum.kt
+++ b/src/main/kotlin/exercise2/task1/FindPairOfHighestSum.kt
@@ -1,5 +1,6 @@
 package exercise2.task1
 
+import exercise2.task2.findHighestSumPairFunctional
 import org.jetbrains.exercise2.common.isEqualsTo
 import org.jetbrains.exercise2.task3.findPairWithBiggestDifference
 
@@ -18,7 +19,20 @@ import org.jetbrains.exercise2.task3.findPairWithBiggestDifference
  */
 
 internal fun List<Int>.findHighestSumPair(): Pair<Int, Int> {
-    TODO("Implement me!!")
+    require(this.size >= 2) { "List must have at least two integers." }
+    require(this.all { it in -1000..1000 })
+    var maxSum = Integer.MIN_VALUE
+    var pair: Pair<Int, Int>? = null
+    for((index, item) in this.dropLast(1).withIndex()) {
+        for(j in (index + 1) until this.size) {
+            val sum = item + this[j]
+            if(sum >= maxSum) {
+                maxSum = sum
+                pair = Pair(item, this[j])
+            }
+        }
+    }
+    return pair!!
 }
 
 fun main() {
diff --git a/src/main/kotlin/exercise2/task2/FindPairOfHighestSumFunctional.kt b/src/main/kotlin/exercise2/task2/FindPairOfHighestSumFunctional.kt
index b1527fa..78a7cc2 100644
--- a/src/main/kotlin/exercise2/task2/FindPairOfHighestSumFunctional.kt
+++ b/src/main/kotlin/exercise2/task2/FindPairOfHighestSumFunctional.kt
@@ -1,5 +1,6 @@
 package exercise2.task2
 
+import exercise2.task1.findHighestSumPair
 import org.jetbrains.exercise2.common.isEqualsTo
 import org.jetbrains.exercise2.task3.findPairWithBiggestDifference
 
@@ -20,7 +21,9 @@ import org.jetbrains.exercise2.task3.findPairWithBiggestDifference
  */
 
 internal fun List<Int>.findHighestSumPairFunctional(): Pair<Int, Int> {
-    TODO("Implement me!!")
+    require(this.size >= 2) { "List must have at least two integers." }
+    require(this.all { it in -1000..1000 })
+    return this.sortedDescending().let { sortedList -> sortedList.first() to sortedList[1] }
 }
 
 fun main() {
diff --git a/src/main/kotlin/exercise2/task3/ConvertMethodToFunctionalApproach.kt b/src/main/kotlin/exercise2/task3/ConvertMethodToFunctionalApproach.kt
index 47f814b..96a5a0b 100644
--- a/src/main/kotlin/exercise2/task3/ConvertMethodToFunctionalApproach.kt
+++ b/src/main/kotlin/exercise2/task3/ConvertMethodToFunctionalApproach.kt
@@ -1,5 +1,6 @@
 package org.jetbrains.exercise2.task3
 
+import exercise2.task2.findHighestSumPairFunctional
 import org.jetbrains.exercise2.common.isEqualsTo
 import kotlin.math.abs
 
@@ -17,24 +18,8 @@ import kotlin.math.abs
  */
 
 internal fun List<Int>.findPairWithBiggestDifference(): Pair<Int, Int> {
-    // TODO refactor me to functional approach and make tests pass!!!
-    var resultPair: Pair<Int, Int>? = null
-    var biggestDifference = Int.MIN_VALUE
-
-    for (i in this.indices) {
-        for (j in (i + 1) until this.size) {
-            val first = this[i]
-            val second = this[j]
-            val absDifference = abs(first - second)
-
-            if (absDifference >= biggestDifference) {
-                biggestDifference = absDifference
-                resultPair = Pair(first, second)
-            }
-        }
-    }
-
-    return resultPair!!
+    require(this.size >= 2) { "List must have at least two integers." }
+    return this.sortedDescending().let { sortedList -> sortedList.first() to sortedList.last() }
 }
 
 fun main() {
diff --git a/src/main/kotlin/exercise2/task4/ProcessCountriesData.kt b/src/main/kotlin/exercise2/task4/ProcessCountriesData.kt
index 291e558..2d29d4c 100644
--- a/src/main/kotlin/exercise2/task4/ProcessCountriesData.kt
+++ b/src/main/kotlin/exercise2/task4/ProcessCountriesData.kt
@@ -59,27 +59,33 @@ internal val countries = listOf(
  */
 
 internal fun List<Country>.findCountryWithBiggestTotalArea(): Country {
-    TODO("Implement me!!!")
+    return this.maxBy { it.totalAreaInSquareKilometers }
 }
 
 internal fun List<Country>.findCountryWithBiggestPopulation(): Country {
-    TODO("Implement me!!!")
+    return this.maxBy { it.population }
 }
 
 internal fun List<Country>.findCountryWithHighestPopulationDensity(): Country {
-    TODO("Implement me!!!")
+    return this.maxBy { it.population / it.totalAreaInSquareKilometers }
 }
 
 internal fun List<Country>.findCountryWithLowestPopulationDensity(): Country {
-    TODO("Implement me!!!")
+    return this.minBy { it.population / it.totalAreaInSquareKilometers }
 }
 
 internal fun List<Country>.findLanguageSpokenInMostCountries(): String {
-    TODO("Implement me!!!")
+    return this.flatMap { it.languages }
+        .groupingBy { it }
+        .eachCount()
+        .toList()
+        .sortedByDescending { it.second }
+        .first()
+        .first
 }
 
 internal fun List<Country>.filterCountriesThatSpeakLanguage(language: String): List<Country> {
-    TODO("Implement me!!!")
+    return this.filter { it.languages.contains(language) }
 }
 
 
diff --git a/src/main/kotlin/exercise2/task5/CreateUserDSL.kt b/src/main/kotlin/exercise2/task5/CreateUserDSL.kt
index 855c0cb..eddbd74 100644
--- a/src/main/kotlin/exercise2/task5/CreateUserDSL.kt
+++ b/src/main/kotlin/exercise2/task5/CreateUserDSL.kt
@@ -49,11 +49,11 @@ internal data class Address(
  */
 
 internal fun user(initUser: User.() -> Unit): User {
-    TODO("Implement me!!!")
+    return User().apply(initUser)
 }
 
 internal fun User.address(initAddress: Address.() -> Unit): User {
-    TODO("Implement me!!!")
+    return apply { address = Address().apply(initAddress) }
 }
 
 fun main() {
diff --git a/src/main/kotlin/exercise3/task1/BalancedBrackets.kt b/src/main/kotlin/exercise3/task1/BalancedBrackets.kt
index 7a6958d..8cef3e1 100644
--- a/src/main/kotlin/exercise3/task1/BalancedBrackets.kt
+++ b/src/main/kotlin/exercise3/task1/BalancedBrackets.kt
@@ -26,7 +26,21 @@ package exercise3.task1
 
 
 internal fun isExpressionBalanced(expression: String): Boolean {
-    TODO("Implement me!!!")
+    val openBrackets = listOf('(', '[', '{')
+    val closedBrackets = listOf(')', ']', '}')
+    var brackets = mutableListOf<Char>()
+
+
+    for(char in expression) {
+        when(char) {
+            in openBrackets -> brackets.add(char)
+            ')' -> if(brackets.isEmpty() || brackets.removeAt(brackets.size - 1) != '(') return false
+            ']' -> if(brackets.isEmpty() || brackets.removeAt(brackets.size - 1) != '[') return false
+            '}' -> if(brackets.isEmpty() || brackets.removeAt(brackets.size - 1) != '{') return false
+        }
+    }
+
+    return brackets.isEmpty()
 }
 
 fun main() {
diff --git a/src/main/kotlin/exercise3/task2/ParenthesesClusters.kt b/src/main/kotlin/exercise3/task2/ParenthesesClusters.kt
index 0e75083..8238567 100644
--- a/src/main/kotlin/exercise3/task2/ParenthesesClusters.kt
+++ b/src/main/kotlin/exercise3/task2/ParenthesesClusters.kt
@@ -24,11 +24,35 @@ package exercise3.task2
  */
 
 internal fun String.splitToBracketsClusters(): List<String> {
-    TODO("Implement me!!!")
+    var clusters =  mutableListOf<String>()
+    var open = 0
+    var builder = StringBuilder()
+    this.forEach {
+        when {
+            it == '(' -> {
+                open++
+                builder.append('(')
+            }
+            it == ')' -> {
+                open--
+                builder.append(')')
+            }
+            else -> throw IllegalArgumentException("Invalid character")
+        }
+
+        if(open == 0) {
+            clusters.add(builder.toString())
+            builder = StringBuilder()
+        }
+
+        if(open < 0) return emptyList()
+    }
+    if(open != 0) return emptyList() else return clusters
 }
 
 fun main() {
     val expressionsToClustersCatalog = mapOf(
+        "()(" to listOf("()"),
         "()()()" to listOf("()", "()", "()"),
         "((()))" to listOf("((()))"),
         "((()))(())()()(()())" to listOf("((()))", "(())", "()", "()", "(()())"),
@@ -41,4 +65,6 @@ fun main() {
             "Expression $expression should be split to $expression clusters, but actual value was $actualClusters."
         }
     }
+
+
 }
diff --git a/src/main/kotlin/exercise3/task3/SherlockValidatesString.kt b/src/main/kotlin/exercise3/task3/SherlockValidatesString.kt
index 55680b8..09a99af 100644
--- a/src/main/kotlin/exercise3/task3/SherlockValidatesString.kt
+++ b/src/main/kotlin/exercise3/task3/SherlockValidatesString.kt
@@ -17,7 +17,7 @@ package exercise3.task3
  * s = abcc
  * This is a valid string because we can remove one `c` and have `1` of each character in the remaining string.
  *
- * s = abccc
+ * s = abcc
  * This string is not valid as we can only remove `1` occurrence of `c`. That leaves character frequencies of
  * {`a`: 1, `b`: 1, `c`: 2}.
  * ```
@@ -29,7 +29,19 @@ package exercise3.task3
  */
 
 internal fun isSherlockValid(s: String): String {
-    TODO("Implement me!!!")
+    require(s.length in 1..100000) { "String length must be in range from 1 to 10^5" }
+    require(s.all{ it in 'a'..'z' }) { "String must only contain small letters (a..z)" }
+
+    val occurrences = s.groupingBy { it }.eachCount().map { it.value }
+    val cMin = occurrences.min()
+    val cMax = occurrences.max()
+
+    return when {
+        cMin == cMax -> "YES"
+        cMax - cMin == 1 && occurrences.count { it == cMax } == 1 -> "YES"
+        cMin == 1 && occurrences.count { it == cMin } == 1 && occurrences.count { it == cMax } == occurrences.size - 1 -> "YES"
+        else -> "NO"
+    }
 }
 
 fun main() {
@@ -38,7 +50,6 @@ fun main() {
     stringsToValidityCatalog.forEach { (string, expectedIsValid) ->
         val actualIsValid = isSherlockValid(string)
         val errorMessageFactory = { answer: String -> if (answer == "YES") "is valid" else "is not valid" }
-
         require(expectedIsValid == actualIsValid) {
             "String \"$string\" is ${errorMessageFactory(expectedIsValid)}," +
                     " but actual value was ${errorMessageFactory(actualIsValid)}."
diff --git a/src/main/kotlin/exercise3/task4/TaxiParkTask.kt b/src/main/kotlin/exercise3/task4/TaxiParkTask.kt
index 0dfe831..3eacacd 100755
--- a/src/main/kotlin/exercise3/task4/TaxiParkTask.kt
+++ b/src/main/kotlin/exercise3/task4/TaxiParkTask.kt
@@ -10,7 +10,9 @@ package exercise3.task4
  * Find all the drivers who performed no trips.
  */
 internal fun TaxiPark.findFakeDrivers(): Set<Driver> {
-    TODO("Implement me!!!")
+    val realDriversNames = this.trips.map { it -> it.driver}.toSet()
+    val allDriversNames = this.allDrivers
+    return allDriversNames.subtract(realDriversNames)
 }
 
 /**
@@ -18,7 +20,14 @@ internal fun TaxiPark.findFakeDrivers(): Set<Driver> {
  * Find all the clients who completed at least the given number of trips.
  */
 internal fun TaxiPark.findFaithfulPassengers(minTrips: Int): Set<Passenger> {
-    TODO("Implement me!!!")
+    //test ovde ne prolazi, nisam siguran zasto
+    return this.trips
+        .flatMap { it.passengers }
+        .groupingBy { it }
+        .eachCount()
+        .filter{ it.value >= minTrips }
+        .map { it.key }
+        .toSet()
 }
 
 /**
@@ -26,7 +35,22 @@ internal fun TaxiPark.findFaithfulPassengers(minTrips: Int): Set<Passenger> {
  * Find all the passengers, who were taken by a given driver more than once.
  */
 internal fun TaxiPark.findFrequentPassengers(driver: Driver): Set<Passenger> {
-    TODO("Implement me!!!")
+    val allPassengers = mutableListOf<Passenger>()
+    this.trips
+        .filter { it.driver == driver }
+        .map { it.passengers }
+        .forEach {
+            it.forEach {
+                allPassengers.add(it)
+            }
+        }
+
+    return allPassengers.groupingBy { it }
+        .eachCount()
+        .filter { it.value > 1 }
+        .map { it.key }
+        .toSet()
+
 }
 
 /**
@@ -34,5 +58,17 @@ internal fun TaxiPark.findFrequentPassengers(driver: Driver): Set<Passenger> {
  * Find the passengers who had a discount for the majority of their trips.
  */
 internal fun TaxiPark.findSmartPassengers(): Set<Passenger> {
-    TODO("Implement me!!!")
+    val passengersMap = trips
+        .flatMap { t -> t.passengers.map { p -> p to (t.discount != null) } }
+        .groupBy({ it.first }, { it.second })
+
+    return passengersMap.filterValues { discounts ->
+            val withDiscount = discounts.count { it }
+            val noDiscount = discounts.size - withDiscount
+            withDiscount > noDiscount
+        }
+        .map { it.key }
+        .toSet()
 }
+
+