3Sum(or k_Sum)

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

Example:

Given array nums = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]
(4Sum问题)
C++:

 class Solution {
 public:
     vector<vector<int>> threeSum(vector<int>& nums) {
         vector<vector<int>> result;
         if (nums.size() <= )return result;
         sort(nums.begin(), nums.end());
         for (int i = ; i < nums.size() - ; i++) {
             int a = nums[i];
             if (a > ) break;
             if (i >  && a == nums[i - ]) continue;
             for (long j = i + , k = nums.size() - ; j < k;) {
                 int b = nums[j];
                 int c = nums[k];
                 int value = a + b + c;
                 if (value == ) {
                     result.push_back(vector<int>({ a, b, c }));
                     while (b == nums[++j] && j<k);
                     while (c == nums[--k] && j<k);
                 }
                 else if (value > ) {
                     k--;
                 }
                 else {
                     j++;
                 }
             }
         }
         return result;
     }
 };