Two Sum

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

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

You can return the answer in any order.

Example 1:

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Explanation: Because nums[0] + nums[1] == 9, we 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]

Constraints:

• 2 <= nums.length <= 104
• -109 <= nums[i] <= 109
• -109 <= target <= 109
• Only one valid answer exists.

Follow-up: Can you come up with an algorithm that is less than O(n2) time complexity?

The code is as follows：

import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

public class P1_TwoSum{
	 public static void main(String[] args) {
	 	int num[]={3,4,6,5};
	 	 //测试代码
	 	 Solution solution = new P1_TwoSum().new Solution();
		 System.out.println(Arrays.toString(solution.twoSum(num,9)));
	 }
	 
    //力扣代码
    class Solution {
        public int[] twoSum(int[] nums, int target) {
            Map<Integer,Integer> flag=new HashMap<>();
            for(int i=0;i< nums.length;i++){
                if(flag.containsKey(target-nums[i])){
                    return new int[]{flag.get(target-nums[i]),i};
                }
                flag.put(nums[i],i);
            }
            return new int[0];
        }
    }
}