1. Sum of two numbers: given an integer array num and an integer target value, please find the two integers whose sum is the target value target in the array and return their array subscripts

subject

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

Ideas

Standard can use double for loop , But the time complexity is relatively high ; So you can Use a tag value , When and are the target content , You can return directly to .

Code

class Solution {
    
    public int[] twoSum(int[] nums, int target) {
    
        Map<Integer,Integer> map = new HashMap<>();
        for(int i=0;i<nums.length;i++){
    
            if(map.containsKey(target - nums[i])){
    
                return new int[]{
    map.get(target - nums[i]),i};
            }
            map.put(nums[i],i);
        }
        return new int[0];
    }
}

Code instructions ：
leetcode The standard answer inside is to use hashTable, such map It's thread safe , But relatively speaking, the performance is poor ;

Map About ： The data in the array and the position in the array where the data is located , Preservation , Set the data to key, The position is set to value; This can ensure that the data in map Do not repeat in
Use one layer for loop effect ： Traverse all the data in the array for In the cycle

if The role of ： Use containsKey Judge the target content （target） Subtract the current loop to data , Whether it already exists map Collection , If it exists, return to the current for Cycle to the coordinates and map The corresponding in the set value, This value is the evaluated value