018.md

15. 4Sum

Solution 1 (O(n^3))

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        sort(nums.begin(), nums.end());
        int length = nums.size();
        vector<vector<int>> ans;
        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 (j > i + 1 && nums[j] == nums[j - 1])
                    continue;
                int p1 = j + 1, p2 = length - 1;
                while (p1 < p2) {
                    int t = nums[i] + nums[j] + nums[p1] + nums[p2];
                    if (t == target) {
                        vector<int> cur;
                        cur.push_back(nums[i]);
                        cur.push_back(nums[j]);
                        cur.push_back(nums[p1]);
                        cur.push_back(nums[p2]);
                        ans.push_back(cur);
                        p1 += 1;
                        p2 -= 1;
                        while (p1 < p2 && nums[p1] == nums[p1 - 1])
                            p1++;
                        while (p1 < p2 && nums[p2] == nums[p2 + 1])
                            p2--;
                    } else if (t < target) {
                        p1++;
                    } else{
                        p2--;
                    }
                }
            }
        }
        return ans;
    }
};