LeetCode: 3Sum

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]]

解析：将3Sum转为2Sum问题，时间复杂度为O(n^2)。注意重复元素。

class Solution {public:    vector<vector<int>> threeSum(vector<int>& nums) {        sort(nums.begin(), nums.end());                vector<vector<int> > res;        if (nums.size() < 3) return res;        int i = 0;        while (i < nums.size() - 2)        {            int left = 0 - nums[i];            int l = i + 1, r = nums.size() - 1;                        while(l < r)            {                int t = nums[l] + nums[r];                if (t == left)                {                    vector<int> tmp;                    tmp.push_back(nums[i]);                    tmp.push_back(nums[l]);                    tmp.push_back(nums[r]);                    res.push_back(tmp);                                        l++;                    r--;                    while(l < r && nums[l] == nums[l-1]) {l++;};                    while(l < r && nums[r] == nums[r+1]){r--;};                }                else if (t < left) {                    l++;                }                else {                    r--;                }            }                        int j = i + 1;            for (j = i + 1; j < nums.size() - 2; ++j) {                if (nums[j] != nums[i]) break;             }                        i = j;        }                return res;            }};