15. 三数之和

解法一：排序+双指针

回溯法做四数之和超时，先来看一下三数之和。
参考官方题解：https://leetcode-cn.com/problems/3sum/solution/san-shu-zhi-he-by-leetcode-solution/

import java.util.*;
class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        final int target = 0;
        final int n = nums.length;
        List<List<Integer>> ans = new LinkedList<>();
        Arrays.sort(nums);
        for (int first = 0; first < n - 2; ) {
            int third = n - 1;
            for (int second = first + 1; (second < n - 1) && (second < third); ) {
                int sum = nums[first] + nums[second] + nums[third];
                while ((sum > target) && (second < third)) {
                    sum -= nums[third];
                    --third;
                    sum += nums[third];
                }
                if (second >= third) {
                    break;
                }
                if (sum == target) {
                    List<Integer> list = new ArrayList<>();
                    list.add(nums[first]);
                    list.add(nums[second]);
                    list.add(nums[third]);
                    ans.add(list);
                }
                ++second;
                while ((nums[second] == nums[second - 1]) && (second < third)) {
                    ++second;
                }
            }
            ++first;
            while ((nums[first] == nums[first - 1]) && (first < n - 2)) {
                ++first;
            }
        }
        return ans;
    }
}