LeetCode-18. 4Sum

Given an array ​​nums​​ of n integers and an integer ​​target​​, are there elements a, b, c, and d in ​​nums​​ such that a + b + c + d = ​​target​​​? Find all unique quadruplets in the array which gives the sum of ​​target​​.

Note:

The solution set must not contain duplicate quadruplets.

Example:

Given array nums = [1, 0, -1, 0, -2, 2], and target = 0.

A solution set is:
[
  [-1,  0, 0, 1],
  [-2, -1, 1, 2],
  [-2,  0, 0, 2]
]

题解：

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