双指针法数组三数之和

15. 三数之和 - 力扣（LeetCode） (leetcode-cn.com)

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        vector<vector<int>> result;
        sort(nums.begin(), nums.end());
    
        // 找出a + b + c = 0    a = nums[i], b = nums[left], c = nums[right]
        for (int i = 0; i < nums.size(); i++) {   //去重原则，先判断了，利用了条件，再下一位碰到同样元素时去重
            // 对a去重
            if(i > 0 && nums[i] == nums[i - 1]) {
                continue;
            }
            int left = i + 1;
            int right = nums.size() - 1;
            while (right > left) {
           
                if (nums[i] + nums[left] + nums[right] > 0) {
                    right--;
                } else if (nums[i] + nums[left] + nums[right] < 0) {
                    left++;
                } else if (nums[i] + nums[left] + nums[right] == 0){
                    result.push_back(vector<int>{nums[i], nums[left], nums[right]});
                    right--;
                    left++;
                    while (right > left && nums[right] == nums[right + 1]) right--;//对b去重
                    while (right > left && nums[left] == nums[left - 1]) left++;//对c去重
                }
            }

        }
        return result;
    }
};

18. 四数之和 - 力扣（LeetCode） (leetcode-cn.com)

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums,int target) {
        vector<vector<int>> result;
        sort(nums.begin(), nums.end());
        for(int k=0;k<nums.size();k++){
            if(k > 0 && nums[k] == nums[k - 1]) {//[2,2,2,2,2]
                continue;
            }
            for (int i = k+1; i < nums.size(); i++) { 
                if(i>k+1 &&nums[i] == nums[i - 1]) {//i>k+1
                    continue;
                }
                int left = i + 1;
                int right = nums.size() - 1;
                while (right > left) {
                    // nums[k] + nums[i] + nums[left] + nums[right] > target 会溢出
                    if (nums[k] + nums[i] > target - (nums[left] + nums[right])) {
                        right--;
        
                    } else if (nums[k] + nums[i]  < target - (nums[left] + nums[right])) {
                        left++;
                    } else if (nums[k]+nums[i] + nums[left] + nums[right] == target){
                        result.push_back(vector<int>{nums[k],nums[i], nums[left], nums[right]});
                        right--;
                        left++;
                        while (right > left && nums[right] == nums[right + 1]) right--;//对b去重
                        while (right > left && nums[left] == nums[left - 1]) left++;//对c去重
                    }
                }
            }
        }
        return result;
    }
};