[LC]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, 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].

思路一：两个for loop，干掉，千万别用。思路二：用map先loop一遍存起来，然后再loop一遍数组，看target-current这个差值存不存在map里。思路二优化：把两次loop合并成一次loop：

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

        return res;
     }