2 Sum ,3 Sum, 3 Sum close

1. Two Sum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

public class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] res = new int[2];
        if(nums == null || nums.length == 0) return res;
        HashMap<Integer, Integer> map = new HashMap<>();
        for(int i = 0; i < nums.length ; i++){
            if(map.containsKey(target-nums[i])){
                res[0] = map.get(target-nums[i]);
                res[1] = i;
            }
            map.put(nums[i], i);
        }
        return res;
    }
}

3 Sum

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note: The solution set must not contain duplicate triplets.

For example, given array S = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

public class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        Arrays.sort(nums);
        for(int i = 0 ; i < nums.length - 2; i++){
            List<Integer> list = new ArrayList<>();
            int left = i + 1;
            int right = nums.length -1;
            int sum =  0 - nums[i];
            while(left < right){
                if(nums[left] + nums[right] == sum){
                    list.add(nums[i]);
                    list.add(nums[left]);
                    list.add(nums[right]);
                    if(!res.contains(list))
                        res.add(new ArrayList<>(list));
                    list.clear();
                    while(left < right && nums[left] == nums[left+1]) left++;
                    while(left < right && nums[right] == nums[right-1]) right--;
                    left++;
                    right--;
                }
                else if(nums[left] + nums[right] < sum){
                    left ++;
                }
                else
                    right--;
            }
        }
        return res;
    }
}

16. 3Sum Closest

Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

    For example, given array S = {-1 2 1 -4}, and target = 1.

    The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

public class Solution {
    public int threeSumClosest(int[] nums, int target) {
      Arrays.sort(nums);
      int min = Integer.MAX_VALUE;
      for(int i = 0 ;  i < nums.length -2; i++){
          int left = i + 1;
          int right = nums.length - 1;
          int value = target - nums[i];
            while(left < right){
                if(nums[left] + nums[right] == value)
                    return target;

                if(min == Integer.MAX_VALUE || Math.abs(nums[i] + nums[left] + nums[right] - target) < Math.abs(min - target))
                    min = nums[i] + nums[left] + nums[right];

                if(nums[left] + nums[right] > value)
                    right --;

                else
                    left++ ;
                }
            }

      return min;
    }
}