15. 3Sum java solutions

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]
]

Subscribe to see which companies asked this question

public class Solution {
    List<List<Integer>> ans = new ArrayList<List<Integer>>();
    public List<List<Integer>> threeSum(int[] nums) {
        if(nums == null || nums.length < 3) return ans;
        Arrays.sort(nums);
        for(int i = 0; i < nums.length - 2; i++){
            if(i > 0 && nums[i] == nums[i-1]) continue;
            find3sum(i+1,nums.length-1,nums,0-nums[i]);
        }
        return ans;
    }
    
    public void find3sum(int s,int e,int[] nums,int target){
        while(s < e){
            int sum = nums[s] + nums[e];
            if(sum == target){
                List<Integer> tmp = new ArrayList<Integer>();
                tmp.add(0-target);
                tmp.add(nums[s++]);
                tmp.add(nums[e--]);
                ans.add(tmp);
                while(s < nums.length-1 && nums[s] == nums[s-1])s++;
                while(e >= 0 && nums[e] == nums[e+1])e--;
            }else if(sum < target) s++;
            else e--;
        }
    }
}

使用求解2sum 的方式来做，要注意的是重复元素的处理。先排序，然后略过重复元素