15. 3Sum (JAVA)

Given an array nums of n integers, are there elements a, b, cin nums 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.

Example:

Given array nums = [-1, 0, 1, 2, -1, -4],

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

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> ret = new ArrayList<>() ;
        if(nums.length==0) return ret;

        int target;
        int len = nums.length-2;
        int left; //point to the left side of the array
        int right; //point to the right side of the array
        
        Arrays.sort(nums);
        
        for(int i = 0; i < len; i++){
            
            target = 0 - nums[i];
            left = i+1;
            right = len+1;
            if(nums[left] > target) break;
            
            while(left < right){
                if(nums[left] + nums[right] > target){
                    right--;
                }
                else if(nums[left] + nums[right] < target){
                    left++;
                }
                else{
                    List<Integer> ans = new ArrayList<>();
                    ans.add(nums[i]);
                    ans.add(nums[left]);
                    ans.add(nums[right]);
                    ret.add(ans);
                    
                    //to avoid IndexOutOfBoundsException
                    left++;
                    right--;
                    //for uniqueness
                    while(nums[left] == nums[left-1] && left < right) left++;
                    while(nums[right] == nums[right+1] && left < right) right--;
                }   
            }
            
            while(nums[i] == nums[i+1]) {
                if(i+1 < len) i++; //for uniqueness
                else return ret;
            }
        }
        return ret;
    }
}

数组问题注意：下标越界

时间复杂度：O(n2)，通过两个指针向中间夹逼的方法使得两个数求和的时间复杂度从O(n2)->O(n)。