15. 3Sum

python:

class Solution(object):    def threeSum(self, nums):        """        :type nums: List[int]        :rtype: List[List[int]]        """        res = []        nums.sort()             #先对数组进行排序        for i in range(len(nums)-2):            if i > 0 and nums[i] == nums[i-1]:               #若当前元素与之前元素相同，不进行之后的操作，更新i=i+1                continue;            #先选中一个元素nums[i],再在之后寻找另外两个元素nums[l],nums[r]            l = i + 1                       r = len(nums) - 1            while l < r:                s = nums[i] + nums[l] + nums[r]                if s < 0:                    l += 1                elif s > 0:                    r -= 1                else:                    res.append((nums[i],nums[l],nums[r]))                    while l < r and nums[l] == nums[l + 1]:                        l += 1                    while l < r and nums[r] == nums[r - 1]:                        r -= 1                    l += 1                    r -= 1        return res

C++:

class Solution {public:     vector<vector<int> > threeSum(vector<int> &num) {        vector<vector<int> > res;        int n = num.size();        sort(num.begin(), num.end());        for(int i = 0; i < n-2; i ++)        {            while(i > 0 && num[i] == num[i-1])                i ++;            int l = i + 1;            int r = n - 1;            while(l < r)            {                int sum = num[i] + num[l] + num[r];                if(sum == 0)                {                    vector<int> cur(3);                    cur[0] = num[i];                    cur[1] = num[l];                    cur[2] = num[r];                    res.push_back(cur);                    l ++;                    r --;                    while(l < r && num[l] == num[l-1])                        l ++;                    while(l > r && num[r] == num[r+1])                        r --;                }                else if(sum < 0)                {                    l ++;                }                else                {                    r --;                }            }        }        return res;    }};