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.

Example:

Given nums = [2, 7, 11, 15], target = 9,Because nums[0] + nums[1] = 2 + 7 = 9,return [0, 1].

UPDATE (2016/2/13):
The return format had been changed to zero-based indices. Please read the above updated description carefully.

Leetcode第一题，用hashmap 可简单解。遍历数组，查找hashmap 里有没有target - nums[i], 有则返回位置，没有则把当前nums[i] 存入hashmap。代码如下：

public class Solution {    public int[] twoSum(int[] nums, int target) {        HashMap<Integer,Integer> ht = new HashMap<Integer,Integer>();        int[] result = new int[2];        ht.put(nums[0], 0);        for (int i = 1; i < nums.length; i++) {            if (ht.containsKey(target - nums[i])) {                result[0] = ht.get(target - nums[i]);                result[1] = i;            } else {                ht.put(nums[i], i);            }        }        return result;    }}