(leetcode) sum of two numbers

Given an array of integers nums  And an integer target value target, Please find... In the array And is the target value target   the   Two   Integers , And return their array subscripts .

You can assume that each input corresponds to only one answer . however , The same element in the array cannot be repeated in the answer .

You can return the answers in any order .

Example 1：

Input ：nums = [2,7,11,15], target = 9
Output ：[0,1]
explain ： because nums[0] + nums[1] == 9 , return [0, 1] .
Example 2：

Input ：nums = [3,2,4], target = 6
Output ：[1,2]
Example 3：

Input ：nums = [3,3], target = 6
Output ：[0,1]
 

Tips ：

2 <= nums.length <= 104
-109 <= nums[i] <= 109
-109 <= target <= 109
There will only be one valid answer
Advanced ： You can come up with a time complexity less than O(n2) The algorithm of ？

Code：

class Solution
{
public:
    //  Input : An array of integers + Integer target 
    //  Output : Subscripts of the two array elements that make up the target integer （ In any order ）
    vector<int> twoSum(vector<int> &nums,int target)
    {
        // unordered_map Implementation based on hash table , The time complexity of searching is low O(1)
        unordered_map<int,int> m;
        // map Based on the red black tree , The time complexity of finding O(n)
        for(int i=0;i<nums.size();i++)
        {
            //  Traverse , If you can form a target integer , Output subscript 
            if(m.count(target-nums[i]))
            {
                return {m[target-nums[i]],i};
            }

            //  Save the data found above unordered_map, In order to match the data not found later 
            m[nums[i]]=i;
        }
        //  If not found, null is returned 
        return {};
    }
};

source ： Power button （LeetCode）
link ：https://leetcode.cn/problems/two-sum

Reference：

C++ map and unordered_map The differences and connections between map Use _m0_67401660 The blog of -CSDN Blog _unordered_map and map The difference between


C++ unordered_map_ The blog of Master Wang who always loves Technology -CSDN Blog _c++ unordered_map