package leetcode.array;

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

/**

* @Author: ChangXuan

* @Decription: 三数之和 https://leetcode-cn.com/problems/3sum/

* @Date: 21:21 2020/10/5

**/

public class ThreeNumberSum {

public static List<List<Integer>> threeSum(int[] nums) {

List<List<Integer>> ans = new ArrayList();

int len = nums.length;

if(nums == null || len < 3) return ans;

Arrays.sort(nums); // 排序

for (int i = 0; i < len ; i++) {

if(nums[i] > 0) break; // 如果当前数字大于0，则三数之和一定大于0，所以结束循环

if(i > 0 && nums[i] == nums[i-1]) continue; // 去重

int L = i+1;

int R = len-1;

while(L < R){

int sum = nums[i] + nums[L] + nums[R];

if(sum == 0){

ans.add(Arrays.asList(nums[i],nums[L],nums[R]));

while (L<R && nums[L] == nums[L+1]) L++; // 去重

while (L<R && nums[R] == nums[R-1]) R--; // 去重

L++;

R--;

}

else if (sum < 0) L++;

else if (sum > 0) R--;

}

}

return ans;

}

}