Basic DSA problems

BinarySearch_Recursive.c

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+//Recursive Binary Search Implementation in C.
+#include <stdio.h>
+int BinarySearch(long long int arr[],long long int low,long long int high,long long int key){
+	if(high < low){
+		return -1;
+	}
+	long long int mid = low + (high-low)/2;
+    if(arr[mid]==key)
+		return mid;
+	else if(key < arr[mid]){
+		return BinarySearch(arr,low,mid-1,key);
+	}
+	else
+		return BinarySearch(arr,mid+1,high,key);
+}
+int main(){
+	long long int n,k,i;
+	scanf("%lld",&n);
+	long long int a[n];
+	for(i=0;i<n;i++){
+		scanf("%lld",&a[i]);
+	}
+	scanf("%lld",&k);
+	long long int b[k];
+	for(i=0;i<k;i++){
+		scanf("%lld",&b[i]);
+	}
+	for(i=0;i<k;i++){
+		b[i] = BinarySearch(a,0,n-1,b[i]);
+		printf("%lld ",b[i]);
+	}
+}

Frequencies_of_limited_range_array.cpp

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+//Frequencies of Limited Range Array Elements
+//Problem link : https://practice.geeksforgeeks.org/problems/frequency-of-array-elements-1587115620/0
+/*
+Given an array A[] of N positive integers which can contain integers from 1 to P where elements can be repeated or can be absent from the array. Your task is to count the frequency of all elements from 1 to N.
+Note: The elements greater than N in the array can be ignored for counting and please read your task section of the problem carefully to understand how to output the array.
+
+
+Example 1:
+
+Input:
+N = 5
+arr[] = {2, 3, 2, 3, 5}
+P = 5
+Output:
+0 2 2 0 1
+Explanation:
+Counting frequencies of each array element
+We have:
+1 occurring 0 times.
+2 occurring 2 times.
+3 occurring 2 times.
+4 occurring 0 times.
+5 occurring 1 time.
+Example 2:
+
+Input:
+N = 4
+arr[] = {3,3,3,3}
+P = 3
+Output:
+0 0 4 0
+Explanation:
+Counting frequencies of each array element
+We have:
+1 occurring 0 times.
+2 occurring 0 times.
+3 occurring 4 times.
+4 occurring 0 times.
+
+Your Task:
+You don't need to read input or print anything. Complete the function frequencycount() that takes the array and n as input parameters and modify the array itself in place to denote the frequency count of each element from 1 to N. i,e element at index 0 should represent the frequency of 1 and so on.
+
+
+Note: Can you solve this problem without using extra space (O(1) Space) !
+
+
+Constraints:
+1 ≤ N ≤ 105
+1 ≤ P ≤ 4*104
+1 <= A[i] <= P
+*/
+
+#include<iostream>
+using namespace std;
+
+int main(){
+
+}//{ Driver Code Starts
+#include<bits/stdc++.h>
+using namespace std;
+
+// } Driver Code Ends
+class Solution{
+    public:
+    //Function to count the frequency of all elements from 1 to N in the array.
+    void frequencyCount(vector<int>& arr,int N, int P)
+    {
+        // code here
+        int hash[N]={0};
+        int i=0;
+        while(i<N){
+            if(arr[i]<=N)
+                hash[arr[i]-1]++;
+            i++;
+        }
+        for(int i=0;i<N;i++){
+            arr[i] = hash[i];
+        }
+    }
+};
+/*
+Time-Complexity : O(N)
+Space-Complexity: O(N)
+*/
+
+//Method-2 : Efficient Method- Since elements of array are positive integers therefore we can count the frequency of occurrence in negative values
+//Algorithm:
+/*
+Traverse the array from start to end.
+For each element check if the element is less than or equal to zero or not. If negative or zero skip the element as it is frequency.
+If an element (e = array[i] – 1 ) is positive, then check if array[e] is positive or not. If positive then that means it is the first occurrence of e in the array and replace array[i] with array[e], i.earray[i] = array[e] and assign array[e] = -1. If array[e] is negative, then it is not the first occurrence, the update array[e] as array[e]– and update array[i] as array[i] = 0.
+Again, traverse the array and print i+1 as value and array[i] as frequency.
+*/
+class Solution{
+    public:
+    //Function to count the frequency of all elements from 1 to N in the array.
+    void frequencyCount(vector<int>& arr,int N, int P)
+    {
+        // code here
+        int i=0;
+        while(i<N){
+            if(arr[i]<=0){
+                i++;
+                continue;
+            }
+            //store the correct index/position of a[i] th element in array to store its frequency of occurrence.
+            int elementIndex = arr[i]-1;
+            //If element at a[i] position is greater than N-1. That means we don't have array size to store its occurrence.
+            if(elementIndex>=N){
+                arr[i]=0;
+                i++;
+            }
+            //If element at correct index/position of arr[i] is greater than 0 that means its not the occurrence frequency count. Its the element for which we have to count occurrence.
+            else if(arr[elementIndex] >0){
+                //If the elementIndex has an element that is not
+                // processed yet, then first store that element
+                // to arr[i] so that we don't lose anything.
+                arr[i] = arr[elementIndex];//There can be some other element at the correct index/position of arr[i] so putting the element arr[elementIndex] at arr[i]
+                // After storing arr[elementIndex], change it
+                // to store initial count of 'arr[i]'
+                arr[elementIndex] = -1;
+            }
+            else{
+                // If this is NOT first occurrence of arr[i],
+                // then decrement its count.
+                arr[elementIndex]--;
+                // And initialize arr[i] as 0 means the element
+                // is not seen so far
+                arr[i] = 0;
+                //Increment the pointer i only when arr[i] is <=0 that means at i th position occurrence of element is calculated.
+                i++;
+            }
+        }
+        //To make all the frequency count to positive numbers as required in output.
+        for(int i=0;i<N;i++){
+            arr[i] = arr[i]*-1;
+        }
+    }
+};
+//Time-Complexity: O(N)
+//Space-Complexity: O(1)
+
+
+//{ Driver Code Starts.
+
+int main()
+{
+	long long t;
+
+	//testcases
+	cin >> t;
+
+	while(t--){
+
+	    int N, P;
+	    //size of array
+	    cin >> N;
+
+	    vector<int> arr(N);
+
+	    //adding elements to the vector
+	    for(int i = 0; i < N ; i++){
+	        cin >> arr[i];
+	    }
+        cin >> P;
+        Solution ob;
+        //calling frequncycount() function
+		ob.frequencyCount(arr, N, P);
+
+		//printing array elements
+	    for (int i = 0; i < N ; i++)
+			cout << arr[i] << " ";
+	    cout << endl;
+	}
+	return 0;
+}
+
+// } Driver Code Ends

Valid_Palindrome.cpp

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+/*
+LeetCode Problem link : https://leetcode.com/problems/valid-palindrome/
+Valid Palindrome
+
+A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.
+
+Given a string s, return true if it is a palindrome, or false otherwise.
+
+
+
+Example 1:
+
+Input: s = "A man, a plan, a canal: Panama"
+Output: true
+Explanation: "amanaplanacanalpanama" is a palindrome.
+Example 2:
+
+Input: s = "race a car"
+Output: false
+Explanation: "raceacar" is not a palindrome.
+Example 3:
+
+Input: s = " "
+Output: true
+Explanation: s is an empty string "" after removing non-alphanumeric characters.
+Since an empty string reads the same forward and backward, it is a palindrome.
+
+
+Constraints:
+
+1 <= s.length <= 2 * 105
+s consists only of printable ASCII characters.
+*/
+#include<bits/stdc++.h>
+using namespace std;
+
+class Solution {
+public:
+    bool isPalindrome(string s) {
+        int l=0,r = s.size()-1;
+        while(l<r){
+            if(!isalnum(s[l])){
+                while(!isalnum(s[l]) && l<r)
+                    l++;
+            }
+            if(!isalnum(s[r])){
+                while(!isalnum(s[r]) && l<r)
+                    r--;
+            }
+
+            if(tolower(s[l])!= tolower(s[r]))
+                return false;
+            l++;
+            r--;
+        }
+        return true;
+    }
+};
+//This is done by me
+int main(){
+    string str = "A man, a plan, a canal: Panama";
+    Solution ob;
+    cout<<ob.isPalindrome(str);
+}
+/*
+Acc. to me
+Time Complexity : O(S), where S is length of string.
+Space Complexity : O(1) since only two extra space is used.
+*/
+
+/*
+Here we have used functions from library <cctype> which is a c style library function.
+isalnum()
+isblank()
+isspace()
+ispunct()
+tolower()
+toupper()
+
+are some functions in this library.
+*/

gcd.c

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+#include <stdio.h>
+int gcd(long int a,long int b){
+	long int c = a%b;
+	if(c!=0){
+		gcd(b,c);
+	}
+	else{
+		printf("%ld",b);
+	}
+}
+int main(){
+	long int a,b;
+	scanf("%ld %ld",&a,&b);
+	gcd(a,b);
+}