leetcode 15. 3Sum

Given an array nums of n integers, are there elements a, b, c in numssuch 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]
]

直接暴力搜索的时间复杂度为$O(N^3)$.

思路：借鉴两数和的思想，先将数组排序，然后双指针。

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        if (nums.size() <= 2)
            return {};
        std::sort(nums.begin(), nums.end());
        vector<vector<int>> res;
        for (int i = 0; i < nums.size() - 2; i++) {
            if (nums[i] > 0) continue; //排好序，如果nums[i]>0,那么后面的两个数加起来不可能小于0
            if ((i > 0) && (nums[i] == nums[i - 1])) continue;
            int li = i + 1, ri = nums.size() - 1, sum = -nums[i];
            while (li < ri) {
                if (nums[li] + nums[ri] == sum) {
                    res.push_back({nums[i], nums[li], nums[ri]});
                    while (li < ri && (nums[li + 1] == nums[li])) li++;
                    while (li < ri && (nums[ri] == nums[ri - 1])) ri--;
                    li++;
                    ri--;
                } else if (nums[li] + nums[ri] < sum) {
                    li++;
                } else {
                    ri--;
                }
            }
        }
        return res;
    }
};