Leetcode 1 Two Sum

Given an array of integers, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution.

Input: numbers={2, 7, 11, 15}, target=9

Output: index1=1, index2=2

Solution 1: The idea is to use hashmap. When iterate through the array, check if target-nums[i] is in the map or not. If in the map, we found the index; if not just put nums[i] as key and index as value in the map and go to next one.

 public class Solution {  
   public int[] twoSum(int[] nums, int target) {  
     Map<Integer,Integer> map=new HashMap<>();  
     for (int i=0; i<nums.length; i++) {  
       int k=target-nums[i];  
       if (map.containsKey(k)) {  
         return new int[] {map.get(k)+1,i+1};  
       }  
       map.put(nums[i],i);  
     }  
     return null;  
   }  
 }