Leetcode(java) 1. Two Sum

Q:

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.

A:

Method 1:

I use list to store the array elments, if list contains (target-current elment), I find the result.

It is necessary to remove the current element from list, otherwise it may lead to mistakes.

class Solution {
    public int[] twoSum(int[] nums, int target) {
        List list = new ArrayList();
        for(int n : nums){
            list.add(n);
        }     
        for(int i=0;i<nums.length;i++){
            list.remove(i);
            if(list.contains(target-nums[i])){    
                return new int[]{i, list.indexOf(target-nums[i])+1};
            }
            list.add(i,nums[i]);
        }

        return new int[2];
    }
}

However, this method takes a long time

Method 2:

In order to save time, I use hash map to search. The map stores the array value and the index.

class Solution {
    public int[] twoSum(int[] nums, int target) {
       HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();

       for(int i=0;i<nums.length;i++){
           if(map.containsKey(nums[i])){
               return new int[]{i,map.get(nums[i])};
           }else{
               map.put(target-nums[i],i);
           }
       }
       return new int[2];
    }
}

result:

This shows Method2 is faster.