15. 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]
]

分析

遍历每一个nums[i], 把它当作 twoSum 的 target number。 39ms

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

class Solution { public:     vector<vector<int>> threeSum(vector<int>& nums) {         int len = nums.size();         vector<vector<int >> result;         if ( len < 3) return result;         sort(nums.begin(), nums.end());         for (int i = 0; i < len - 2; i++){             if( i > 0 && nums[i] == nums [ i - 1]) continue;             /** cut edge to accelerate the speed **/             if( nums[i] + nums[i + 1] + nums[i + 2] > 0) break;             if( nums[i] + nums[len - 2] + nums[ len - 1] < 0) continue;                           int l = i + 1, r = len - 1;             while ( l < r){                 int sum = nums[i] + nums[l] + nums[r];                 if( sum < 0) l++;                 else if( sum > 0) r--;                 else {                     result.push_back(vector<int>{nums[i], nums[l], nums[r]});                     /** cut edge to accelerate the speed **/                     l++;r--;                     while(l < r && nums[l] == nums[l - 1])l++;                     while(l < r && nums[r] == nums[r + 1])r--;                 }             }         }         return result;     } };

来自为知笔记(Wiz)