Leetcode 18. 4Sum

文章作者：Tyan
博客：noahsnail.com  |  CSDN  |  简书

1. Description

4Sum

2. Solutions

• Brute Force

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        int length = nums.size();
        vector<vector<int>> result;
        sort(nums.begin(), nums.end());
        for(int i = 0; i < length - 3; i++) {
            if(i != 0 && nums[i] == nums[i - 1]) {
                continue;
            }
            for(int j = i + 1; j < length - 2; j++) {
                if(nums[j] == nums[j - 1] && j != i + 1) {
                    continue;
                }
                for(int k = j + 1; k < length - 1; k++) {
                    if(nums[k] == nums[k - 1] && k != j + 1) {
                        continue;
                    }
                    for(int l = k + 1; l < length; l++) {
                        if(nums[l] == nums[l - 1] && l != k + 1) {
                            continue;
                        }
                        int sum = nums[i] + nums[j] + nums[k] + nums[l];
                        if(sum == target) {
                            vector<int> temp;
                            temp.push_back(nums[i]);
                            temp.push_back(nums[j]);
                            temp.push_back(nums[k]);
                            temp.push_back(nums[l]);
                            result.push_back(temp);
                        }
                    }
                }
            }
        }
        return result;
    }
};

• sort and O(n^3)

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        int length = nums.size();
        vector<vector<int>> result;
        sort(nums.begin(), nums.end());
        int sum = 0;
        int newTarget = 0;
        int left = 0;
        int right = 0;
        for(int i = 0; i < length - 3; i++) {
            if(i != 0 && nums[i] == nums[i - 1]) {
                continue;
            }
            for(int j = i + 1; j < length - 2; j++) {
                if(nums[j] == nums[j - 1] && j != i + 1) {
                    continue;
                }
                newTarget = target - nums[i] - nums[j];
                left = j + 1;
                right = length - 1;
                while(left < right) {
                    if(left != j + 1 && nums[left] == nums[left - 1]) {
                        left++;
                        continue;
                    }
                    if(right < length - 2 && nums[right] == nums[right + 1]) {
                        right--;
                        continue;
                    }
                    sum = nums[left] + nums[right];
                    if(sum == newTarget) {
                        vector<int> temp;
                        temp.push_back(nums[i]);
                        temp.push_back(nums[j]);
                        temp.push_back(nums[left]);
                        temp.push_back(nums[right]);
                        result.push_back(temp);
                        left++;
                        right--;
                    }
                    else if(sum < newTarget) {
                        left++;
                    }
                    else {
                        right--;
                    }
                }
                
            }
        }
        return result;
    }
};

Reference

1. https://leetcode.com/problems/4sum/description/