Two pointer

Problem1.java

@@ -0,0 +1,24 @@
+class Problem1 {
+    public void sortColors(int[] nums) {
+        int low = 0, high = nums.length-1, mid = 0;
+
+        while(mid <= high){
+            if(nums[mid] == 2){
+                swap(nums, mid, high);
+                high--;
+            }else if(nums[mid] == 0){
+                swap(nums, mid, low);
+                low++;
+                mid++;
+            }else{
+                mid++;
+            }
+        }
+    }
+
+    private void swap(int[] nums, int x, int y){
+        int temp = nums[x];
+        nums[x] = nums[y];
+        nums[y] = temp;
+    }
+}

Problem2.java

@@ -0,0 +1,40 @@
+class Problem2 {
+    public List<List<Integer>> threeSum(int[] nums) {
+        Arrays.sort(nums);
+        int n = nums.length;
+        int target = 0;
+        List<List<Integer>> result = new ArrayList<>();
+        for(int i=0; i<n; i++){
+
+            if(i != 0 && nums[i] == nums[i-1]) continue;
+
+            int innerTarget = target - nums[i];
+
+            for(int j=i+1; j<n; j++){
+
+                if(j != i+1 && nums[j] == nums[j-1]) continue;
+
+                int t = innerTarget - nums[j];
+                int idx = binarySearch(nums, j+1, nums.length-1, t);
+                if(idx != -1){
+                    List<Integer> triplet = Arrays.asList(nums[i], nums[j], nums[idx]);
+                    result.add(triplet);
+                }
+            }
+        }
+
+        return result;
+    }
+
+    private int binarySearch(int[] nums, int low, int high, int target){
+
+        while(low <= high){
+            int mid = low + (high - low)/2;
+            if(nums[mid] == target) return mid;
+            else if(nums[mid] > target) high = mid - 1;
+            else low = mid + 1;
+        }
+
+        return -1;
+    }
+}

Problem3.java

@@ -0,0 +1,25 @@
+class Problem3 {
+    public int maxArea(int[] height) {
+        int n = height.length;
+
+        int low = 0, high = n-1;
+        int area = 0;
+
+        while(low < high){
+            int h = 0;
+            int w = high - low;
+
+            if(height[low] < height[high]){
+                h = height[low];
+                low++;
+            }else{
+                h = height[high];
+                high--;
+            }   
+
+            area = Math.max(area, h*w);
+        }
+
+        return area;
+    }
+}