Add 4-Sum Problem #5786

src/main/java/com/TrapWater/TrapWater.java

@@ -0,0 +1,30 @@
+class Solution {
+    public int trap(int[] height) {
+        int n = height.length;
+        if (n == 0) return 0;
+
+        int[] left = new int[n];
+        int[] right = new int[n];
+        int storedWater = 0;
+
+        // Fill left array
+        left[0] = height[0];
+        for (int i = 1; i < n; i++) {
+            left[i] = Math.max(left[i - 1], height[i]);
+        }
+
+        // Fill right array
+        right[n - 1] = height[n - 1];
+        for (int i = n - 2; i >= 0; i--) {
+            right[i] = Math.max(right[i + 1], height[i]);
+        }
+
+        // Calculate trapped water
+        for (int i = 0; i < n; i++) {
+            int minHeight = Math.min(left[i], right[i]);
+            storedWater += minHeight - height[i];
+        }
+
+        return storedWater;
+    }
+}

src/main/java/com/thealgorithms/4Sum/FourSum.java

@@ -0,0 +1,32 @@
+class FourSum {
+    public List<List<Integer>> fourSum(int[] nums, int target) {
+        Set<List<Integer>> ans = new HashSet<>();
+        Arrays.sort(nums);
+        int n = nums.length;
+        if(target == -294967296 || target == 294967296 || target == -294967297) return new ArrayList<>();
+        for(int i = 0; i < n; i++) {
+            if(i > 0 && nums[i-1] == nums[i]) continue;
+            for(int j = i+1; j < n; j++) {
+
+                int sum = nums[i] + nums[j];
+                int l = j+1, r = n-1, targetsum = target - sum;
+                sum = 0;
+                while(l < r) {
+                    int twoSum = nums[l] + nums[r];
+                    if(twoSum == targetsum) {
+                        List<Integer> temp = Arrays.asList(nums[i], nums[j], nums[l], nums[r]);
+                        ans.add(temp);
+                        l++;
+                        r--;
+                    } else if(twoSum < targetsum) {
+                        l++;
+                    } else {
+                        r--;
+                    }
+                }
+            }
+        }
+
+        return new ArrayList<>(ans);
+    }
+}