1.Two Sum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

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

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

给定一个整数数列，找出其中和为特定值的那两个数。

你可以假设每个输入都只会有一种答案，同样的元素不能被重用。

示例:

给定 nums = [2, 7, 11, 15], target = 9

因为 nums[0] + nums[1] = 2 + 7 = 9
所以返回 [0, 1]

Java：

 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(target - nums[i]))
            {
                return new int[] {i ,map.get(target - nums[i]).intValue()};
            }
            map.put(nums[i],i);
        }
        throw new IllegalArgumentException("No two sum solution");
    }
}