LeetCode 刷题【15. 三数之和】

15. 三数之和

自己做

解1：三层for循环（超时）

cpp 复制代码

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        int len = nums.size();

        vector<vector<int>> res;

        for (int i = 0; i < len; i++)
            for (int j = i + 1; j < len; j++)
                for (int k = j + 1; k < len; k++)
                    if (i != j && i != k && j != k && (nums[i] + nums[j] + nums[k] == 0)) {
                        int res_len = res.size();
                        if (res_len > 0) {
							bool unique = true; //标记是否唯一
                            for (int r = 0; r < res_len; r++)                       //检查是否重复
                                if (res[r][0] == nums[i] && res[r][1] == nums[j] && res[r][2] == nums[k] ||          //重复
                                    res[r][0] == nums[i] && res[r][1] == nums[k] && res[r][2] == nums[j] ||
                                    res[r][0] == nums[j] && res[r][1] == nums[k] && res[r][2] == nums[i] ||
                                    res[r][0] == nums[j] && res[r][1] == nums[i] && res[r][2] == nums[k] ||
                                    res[r][0] == nums[k] && res[r][1] == nums[i] && res[r][2] == nums[j] ||
                                    res[r][0] == nums[k] && res[r][1] == nums[j] && res[r][2] == nums[i]
                                    )
									unique = false; //如果有重复则标记为false
                            if(unique)          //无重复
                                res.push_back(vector<int>({ nums[i] , nums[j] , nums[k] }));
                        }
                        else {                                                        //首个三元组不考虑
                            res.push_back(vector<int>({ nums[i] , nums[j] , nums[k] }));
                        }
                    }

        return res;
    }
};

解2：两层for+哈希（超时）

cpp 复制代码

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        int len = nums.size();
        map<int, int> m;
        vector<vector<int>> res;

        for (int i = 0; i < len; i++)
            m.insert(make_pair(nums[i], i));

        for (int i = 0; i < len; i++)
            for (int j = i + 1; j < len; j++) {
                map<int,int>::iterator num3 = m.find(-(nums[i] + nums[j]));
                if (num3 != m.end()) {                 //找到相匹配的的num3 => num1 + num2 = - num3
                    int k = num3->second;
                    //检查是否存在重复
                    if (i != k && j != k) {
                        int res_len = res.size();
                        if (res_len > 0) {
                            bool unique = true; //标记是否唯一
                            for (int r = 0; r < res_len; r++)                       //检查是否重复
                                if (res[r][0] == nums[i] && res[r][1] == nums[j] && res[r][2] == nums[k] ||          //重复
                                    res[r][0] == nums[i] && res[r][1] == nums[k] && res[r][2] == nums[j] ||
                                    res[r][0] == nums[j] && res[r][1] == nums[k] && res[r][2] == nums[i] ||
                                    res[r][0] == nums[j] && res[r][1] == nums[i] && res[r][2] == nums[k] ||
                                    res[r][0] == nums[k] && res[r][1] == nums[i] && res[r][2] == nums[j] ||
                                    res[r][0] == nums[k] && res[r][1] == nums[j] && res[r][2] == nums[i]
                                    )
                                    unique = false; //如果有重复则标记为false
                            if (unique)          //无重复
                                res.push_back(vector<int>({ nums[i] , nums[j] , nums[k] }));
                        }
                        else {                                                        //首个三元组不考虑
                            res.push_back(vector<int>({ nums[i] , nums[j] , nums[k] }));
                        }
                    }
                    
                }
            }



        return res;
    }
};

看题解

双指针化两层for循环为一层

cpp 复制代码

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        int len = nums.size();
        vector<vector<int>> res;

        //排序数组
        sort(nums.begin(), nums.end());

        for (int first = 0; first < len; first++) {
            int third = len - 1;                                    //右指针【指向最大值】
            
            while (first < len && first > 0 && nums[first] == nums[first - 1]) {            //上一回相等的情况下，为防止撞车，所以first不能等于原来的值
                first++;
            }

            for (int second = first + 1; second < third; second++) {

                while (second < third && second > first + 1 && nums[second] == nums[second - 1]) {            //上一回相等的情况下，为防止撞车，所以second不能等于原来的值
                    second++;
                }

                while(second < third && !(nums[first] + nums[second] + nums[third] == 0)){
                    if (second < third && nums[first] + nums[second] + nums[third] > 0) {                      //右指针third大了
                        third--;
                    }

                    if (second < third && nums[first] + nums[second] + nums[third] < 0) {                      //左指针second小了
                        second++;
                    }
                }

                //找到nums[first] + nums[second] + nums[third] = 0，将结果存放
                if (second < third && nums[first] + nums[second] + nums[third] == 0)
                    res.push_back(vector<int>({ nums[first], nums[second], nums[third] }));
                
            }

        }

        return res;
    }
};