Completed Two-Pointers-1 Assignment

3sum.java

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+// Time Complexity : O(n^2)
+// Space Complexity : O(1)
+// Did this code successfully run on Leetcode : Yes
+// Explaination : We use a two-pointer approach after sorting the array to find all unique triplets that sum to zero.
+
+class Solution {
+    public List<List<Integer>> threeSum(int[] nums) {
+        Arrays.sort(nums);
+        int n = nums.length;
+        List<List<Integer>> result = new ArrayList<>();
+        for(int i=0;i<n;i++){
+            if(i!=0 && nums[i]==nums[i-1]) continue;
+            if(nums[i]>0) break;
+            int low=i+1, high= n-1;
+            while(low<high){
+                int sum = nums[i]+nums[low]+nums[high];
+                if(sum==0){
+                    result.add(Arrays.asList(nums[i],nums[low],nums[high]));
+                    low++;
+                    high--;
+                    while(low<high && nums[low]==nums[low-1]) low++;
+                    while(low<high && nums[high]==nums[high+1]) high--;
+                }
+                else if(sum>0) high--;
+                else low++; 
+            }
+        }
+        return result;
+    }
+}

container_most_water.java

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+// Time Complexity : O(n)
+// Space Complexity : O(1)
+// Did this code successfully run on Leetcode : yes 
+// Explaination : We use two pointers to calculate the area and 
+// move the pointer with smaller height to find the maximum area.
+class Solution {
+    public int maxArea(int[] height) {
+        int n = height.length;
+        int l =0;
+        int r = n-1;
+        int max=0;
+        while(l<r){
+            int curr = Math.min(height[l],height[r]);
+            int area = curr*(r-l);
+            max= Math.max(max,area);
+            if(height[l]<=height[r]){
+                l++;
+            }else{
+                r--;
+            }
+        }
+        return max;
+    }
+}

sort_colors.java

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+// Time Complexity : O(n)
+// Space Complexity : O(1)
+// Did this code successfully run on Leetcode : Yes
+// Explaination : We use three pointers to sort the array in-place.
+class Solution {
+    public void sortColors(int[] nums) {
+        int high = nums.length-1;
+        int low = 0;
+        int 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 m, int l){
+        int temp;
+        temp = nums[m];
+        nums[m]= nums[l];
+        nums[l]= temp;
+    }
+}