Competitive-Coding- 3 complete

KDiffPairsInAnArray.java

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+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * I took frequencyMap to maintain the count of occurences of a num in nums.
+ * I iterated on the frequencyMap to avoid finding repeated pairs.
+ * Considering the key as nums[i], I am looking for nums[j] which is equal to nums[i] - k.
+ * nums[j] can also be equal to nums[i] + k. But I'm not looking for it to avoid finding repeated pairs.
+ * When nums[i] - k is found I am increasing the result by 1.
+ * When k = 0, I need look for repeated numbers since nums[i] - nums[j] = 0 => nums[i] = nums[j].
+ */
+// Time: N + N ~= O(N)
+// Space: O(N) for frequencyMap
+class Solution {
+    public int findPairs(int[] nums, int k) {
+        Map<Integer, Integer> frequencyMap = new HashMap<>();
+        // Populate map
+        for (int num: nums) {
+            frequencyMap.put(num, frequencyMap.getOrDefault(num, 0) + 1);
+        }
+        int numOfPairs = 0;
+        // Iterate through key set of the map
+        for (Map.Entry<Integer, Integer> entry: frequencyMap.entrySet()) {
+            if (k != 0) {
+                if (frequencyMap.containsKey(entry.getKey() - k)) {
+                    numOfPairs ++;
+                }
+            } else {
+                // As k = 0, look for repeated number
+                if (entry.getValue() >= 2) {
+                    numOfPairs ++;
+                }
+            }
+        }
+        return numOfPairs;
+    }
+}

PascalsTriangle.java

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+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+/**
+ * I initalized the first row and second row.
+ * Iterated through each index of the next rows. Initialized values first and last indices of each row as 1.
+ * Calculated the remaining values using the formula triangle.get(rowNum - 1).get(index - 1) + triangle.get(rowNum - 1).get(index)
+ */
+// Time: O(N * (N + 1) / 2) ~= O(N^2); where N = numRows
+// Space: O(N) for currentRow inside for loop
+class Solution {
+    public List<List<Integer>> generate(int numRows) {
+        List<List<Integer>> triangle = new ArrayList<>();
+        triangle.add(Arrays.asList(1)); // first row
+        if (numRows == 1) {
+            return triangle;
+        }
+        triangle.add(Arrays.asList(1, 1)); // second row
+        // Iterate from third row to the end
+        for (int rowNum = 2; rowNum < numRows; rowNum ++) {
+            List<Integer> currentRow = new ArrayList<>();
+            currentRow.add(1); // first element
+            for (int index = 1; index < rowNum; index ++) {
+                currentRow.add(index,
+                    triangle.get(rowNum - 1).get(index - 1) + triangle.get(rowNum - 1).get(index));
+            }
+            currentRow.add(1); // last element
+            triangle.add(currentRow);
+        }
+        return triangle;
+    }
+}