LeetCode01-two sum

题目链接地址：https://leetcode.com/problems/two-sum/

题目详情：

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

题目解析：

思路一：采用遍历的方式对每一个元素相加得到target,时间复杂度O(n^2),但是采用java时，这种方法会超时。

思路二：用hashmap，将每个元素值作为key，数组索引作为value存入hashmap，然后遍历数组元素，在hashmap中寻找与之和为target的元素。 时间复杂度O(n)，空间复杂度O(n)。

对于思路一，采用java时超时，不予采用

对于思路二，详细代码如下：

 public static int[] twoSum2(int[] numbers, int target) {        int[] index=new int[2];        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+1);        System.out.println("numbers[i]:"+(target-numbers[i])+","+map.get(target-numbers[i]));        }else{        System.out.println("numbers[i]:"+numbers[i]+","+map.get(numbers[i]));        index[0]=map.get(numbers[i]);        index[1]=i+1;        System.out.println("index[0]"+index[0]);        System.out.println("index[1]"+index[1]);        }        }        return index;        }