感受到了自己與大神的差距，同樣的題，我的程式碼跑140ms，大神的只要4ms。

題目：

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9, return [0, 1].

我的程式碼(暴力遍歷)：

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        int first=-1,second=-1;
    for(int i=0;i<nums.size();++i){
        //if(nums[i]<target){
            for(int j=0;j<i;++j){
                if(i!=j && nums[i]+nums[j]==target){
                    if(i>j){
                        first=j;second=i;
                    }
                    if(i<j){
                        first=i;second=j;
                    }
                }
        //    }
        }
    }
    return vector<int>{first,second};

    }
};
 

時間複雜度O(n²) 思路：兩層迴圈，每兩個都配對嘗試一遍，忽略掉index相同的部分。

大神的程式碼(One-pass Hash Table)：

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) 
    {
        unordered_map<int, int> map;
        
        for (int i = 0; i < nums.size(); i++)
        {
            int complement = target - nums[i];
            if (map.find(complement) != map.end() && map[complement] != i)
            {
                return vector<int> {map[complement], i};
            }
            map.emplace(nums[i], i);
        }
    }
};
 

時間複雜度O(n) 思路：建立一個hashTable，每次將資料放入前檢查hashTable中是否已有當前要放入數的補數。

測試程式：

#include<iostream>
#include<vector>
#include<map>
#include<unordered_map>
#include<algorithm>
using namespace std;

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target)
    {
        unordered_map<int, int> map;

        for (int i = 0; i < nums.size(); i++)
        {
            int complement = target - nums[i];
            if (map.find(complement) != map.end() && map[complement] != i)
            {
                return vector<int> {map[complement], i};
            }
            map.emplace(nums[i], i);
        }
    }
};
//判斷是否合格的巨集
#define REQUIRE(sol) {\
    if(sol)\
    cout<<"passed"<<endl;\
    else cout<<"not passed"<<endl;\
}

int main(){

    Solution s;
    //測試集
    std::vector<int> v1{2, 7, 11, 15};
    REQUIRE( (s.twoSum(v1, 9) == std::vector<int>{0, 1}) );

    std::vector<int> v2{0, 4, 3, 0};
    REQUIRE( (s.twoSum(v2, 0) == std::vector<int>{0, 3}) );

    std::vector<int> v3{-3, 4, 3, 90};
    REQUIRE( (s.twoSum(v3, 0) == std::vector<int>{0, 2}) );

    std::vector<int> v4{2, 15, 11, 7};
    REQUIRE( (s.twoSum(v4, 9) == std::vector<int>{0, 3}) );

    return 0;
}

炫一下自己的提交：

Time Submitted Status Runtime Language 13 hours, 14 minutes ago [**Accepted**](https://leetcode.com/submissions/detail/177511062/) 4 ms cpp 13 hours, 15 minutes ago [**Accepted**](https://leetcode.com/submissions/detail/177511034/) 8 ms cpp 13 hours, 17 minutes ago [**Accepted**](https://leetcode.com/submissions/detail/177510934/) 8 ms cpp 13 hours, 34 minutes ago [**Accepted**](https://leetcode.com/submissions/detail/177509923/) 12 ms cpp 14 hours, 6 minutes ago [**Accepted**](https://leetcode.com/submissions/detail/177508255/) 4 ms cpp 14 hours, 31 minutes ago [**Accepted**](https://leetcode.com/submissions/detail/177506900/) 324 ms cpp 18 hours, 5 minutes ago [**Accepted**](https://leetcode.com/submissions/detail/177485296/) 140 ms cpp 324ms那個是自己寫了一個根據value查詢key的函式，結果效率奇底。

12ms那個是用的map，沒有使用unordered_map，所以慢了點。