Two sum 两个数相加

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].
第一种：暴力
第二种：排序，然后找
第三种：哈希表法

java：第一遍将数字变为哈希表的key，第二遍直接找到哈希表中的key是否存在。

public static int[] twoSum1(int[] nums, int target){
         //基于哈希表
        int j;
        int i;
        HashMap<Integer,Integer> mymap = new HashMap<Integer, Integer>();
        for( i = 0 ; i < nums.length ;i ++){
            mymap.put(nums[i],i);
        }
        for( i = 0 ; i < nums.length ; i++){
            j = target - nums[i];

            if(mymap.get(j)!=null && mymap.get(j)!=i){
                int arr[] = new int[2];
                arr[0] = i;
                arr[1] = mymap.get(j);
                return arr;
            }
        }

        return null;
    }

java 更好的写法2：

一边放入哈希表一边看是否已经存在答案

int j;
        int i;
        int arr[] = new int[2];
        HashMap<Integer,Integer> mymap = new HashMap<Integer, Integer>();
        for( i = 0 ; i < nums.length ;i ++){
            if(mymap.containsKey(target - nums[i])){
                arr[0] = i;
                arr[1] = mymap.get(target - nums[i]);
                return arr;
            }
            mymap.put(nums[i],i);
        }


        return null;