跟我一起学算法系列1---Two Sum（java）

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].

题意：
一个数组中两个位置上的数的和恰为 target，求这两个位置。

解法：
可以用 Map 记录出现的数，只要判断有没有和当前的数凑成 target 的数，再找出来即可。

实现：

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