15. 三数之和 - 力扣（LeetCode） (leetcode-cn.com)

 1 class Solution {
 2 public:
 3     vector<vector<int>> threeSum(vector<int>& nums) {
 4         vector<vector<int>> result;
 5         sort(nums.begin(), nums.end());
 6     
 7         // 找出a + b + c = 0    a = nums[i], b = nums[left], c = nums[right]
 8         for (int i = 0; i < nums.size(); i++) {   //去重原则，先判断了，利用了条件，再下一位碰到同样元素时去重
 9             // 对a去重
10             if(i > 0 && nums[i] == nums[i - 1]) {
11                 continue;
12             }
13             int left = i + 1;
14             int right = nums.size() - 1;
15             while (right > left) {
16            
17                 if (nums[i] + nums[left] + nums[right] > 0) {
18                     right--;
19                 } else if (nums[i] + nums[left] + nums[right] < 0) {
20                     left++;
21                 } else if (nums[i] + nums[left] + nums[right] == 0){
22                     result.push_back(vector<int>{nums[i], nums[left], nums[right]});
23                     right--;
24                     left++;
25                     while (right > left && nums[right] == nums[right + 1]) right--;//对b去重
26                     while (right > left && nums[left] == nums[left - 1]) left++;//对c去重
27                 }
28             }
29 
30         }
31         return result;
32     }
33 };

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