LeetCode 18 4Sum

LeetCode 18 4Sum

思路

本题就是在3Sum的外面再套上一个循环即可。

代码

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        int subtarget, sum;
        vector<vector<int>> rs;
        sort(nums.begin(), nums.end());
        
        for (int i = 0; i < nums.size(); ++i) {
            subtarget = target - nums[i];
            for (int j = i+1; j < nums.size(); ++j) {
                int left = j + 1, right = nums.size() - 1;
                while (left < right) {
                    sum = nums[j] + nums[left] + nums[right];
                    if (sum < subtarget) ++left;
                    else if (sum > subtarget) --right;
                    else {
                        rs.emplace_back(vector<int>{nums[i], nums[j], nums[left], nums[right]});
                        while (left < right && nums[left + 1] == nums[left]) ++left;
                        while (left < right && nums[right -1] == nums[right]) --right;
                        ++left;
                        --right;
                    }
                }
                
                while (j + 1 < nums.size() && nums[j+1] == nums[j]) ++j;
            }
            
            while (i + 1 < nums.size() && nums[i+1] == nums[i]) ++i;
        }
        
        return rs;
    }
};