Leetcode top 100 questions 1 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 .

class Solution {
    
    public int[] twoSum(int[] nums, int target) {
    
        for(int i=0;i<nums.length;i++){
    
            for(int j=i+1;j<nums.length;j++){
    
                if(nums[i]+nums[j]==target) {
    
                    retrun new int[]{
    i,j};
                }
            }
        }
        return new int[0];
    }
}

This method is very simple , Direct two nested traversal , You can make a judgment every time you traverse , But the time complexity is high ,O(n^2)

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

You can also use hash Do it with a watch , use hashMap.containsKey() Methods to determine , Whether it conforms to key stay hash In the table , among key Is to use the value of the array ,value Use array subscripts .