18. 4Sum

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18. 4Sum

Description

Difficulty: Medium

Related Topics: Array, Two Pointers, Sorting

Given an array nums of n integers, return an array of all the unique quadruplets [nums[a], nums[b], nums[c], nums[d]] such that:

  • 0 <= a, b, c, d < n
  • a, b, c, and d are distinct.
  • nums[a] + nums[b] + nums[c] + nums[d] == target

You may return the answer in any order.

Example 1:

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Input: nums = [1,0,-1,0,-2,2], target = 0
Output: [[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]

Example 2:

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Input: nums = [2,2,2,2,2], target = 8
Output: [[2,2,2,2]]

Constraints:

  • 1 <= nums.length <= 200
  • -109 <= nums[i] <= 109
  • -109 <= target <= 109

Hints/Notes

  • Use nSum approach next time

Solution

Language: C++

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class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        sort(nums.begin(), nums.end());
        vector<vector<int>> res;
        for (int i = 0; i < nums.size(); i++) {
            threeSum(res, nums, i, target - nums[i]);
            while (i < nums.size() - 1 && nums[i] == nums[i + 1]) i++;
        }
        return res;
    }

    void threeSum(vector<vector<int>> &res, vector<int> &nums, int index, long target) {
        for (int i = index + 1; i < nums.size(); i++) {
            twoSum(res, nums, nums[index], i, target - nums[i]);
            while (i < nums.size() - 1 && nums[i] == nums[i + 1]) i++;
        }
    }

    void twoSum(vector<vector<int>> &res, vector<int> &nums, int num1, int index, long target) {
        int left = index + 1, right = nums.size() - 1;
        while (left < right) {
            int lvalue = nums[left], rvalue = nums[right], sum = nums[left] + nums[right];
            if (sum > target) {
                while (left < right && nums[right] == rvalue) right--;
            } else if (sum < target) {
                while (left < right && nums[left] == lvalue) left++;
            } else {
                res.push_back({num1, nums[index], lvalue, rvalue});
                while (left < right && nums[left] == lvalue) left++;
                while (left < right && nums[right] == rvalue) right--;
            }
        }
    }
};