[leetcode 1] Two Sum

1 题目

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

2 方法

循环遍历两遍，系统显示时间超了。提示使用hashmap。

那么时间就是O(n)，空间也是O(n)。

3 代码

参考了 http://www.xuebuyuan.com/1547526.html

public int[] twoSum(int[] numbers,int target){    
        int[] indexs = {0,0};
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int i = 0; i < numbers.length; i++) {
            if(!map.containsKey(numbers[i])){
                map.put(target-numbers[i],i);
            }else{
                indexs[0] = map.get(numbers[i]) + 1;
                indexs[1] = i + 1;
            }
        }
        return indexs;
｝