【Hot100】1. Sum of two numbers

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

solution ：
We need a method that can quickly find whether the target element exists in the array . If there is , Need to find its index .

Use hash table , You can look for target - nums[i] The time complexity is reduced to from O(N) Down to O(1).

Create a Hashtable , For each of these nums[i], First, query the hash table to see if target - nums[i], And then nums[i] Insert into hash table , You can guarantee that you won't let nums[i] Match yourself .
Points of attention ：
①map：
key-> Array value
val-> Subscript

② Already exist map The subscript in must be in front , therefore j When the result is returned to the array, put map Put the subscript in the front .

③ return null Two ways of writing ：
return null;
return new int[0];

Code ：

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

        }
        return new int[0];

    }
}