leetcode 3Sum

题目连接

https://leetcode.com/problems/3sum/ 

3Sum

Description

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: 
Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c) 
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)
3sum问题参考链接：https://en.wikipedia.org/wiki/3SUM

class Solution {
public:
	vector<vector<int>> threeSum(vector<int>& nums) {
		typedef vector<int> vec;
		typedef vector<vec> mat;
		mat ans;
		size_t n = nums.size();
		if (n < 3) return ans;
		sort(nums.begin(), nums.end());
		for (size_t i = 0; i < n - 2; i++) {
			if (i > 0 && nums[i] == nums[i - 1]) continue;
			size_t j = i + 1, k = n - 1;
			while (j < k) {
				int sum = nums[i] + nums[j] + nums[k];
				if (!sum) {
					if (j > i + 1 && nums[j] == nums[j - 1]) {
						j++;
						continue;
					}
					vec ret;
					ret.push_back(nums[i]);
					ret.push_back(nums[j]);
					ret.push_back(nums[k]);
					ans.push_back(ret);
					j++;
				}
				if (sum > 0) {
					k--;
					if (nums[k] == nums[k + 1]) k--;
				}
				if (sum < 0) {
					j++;
					if (nums[j] == nums[j - 1]) j++;
				}
			}
		}
		return ans;
	}
};