Two Sum

题目

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

返回符合题意 two integers 在数组中的位置（从0开始数起）

HashMap 查找元素时复是O（1）

思路

1. there could be various followups for this question

图解

代码

class Solution {
    public int[] twoSum(int[] nums, int target) {
        HashMap<Integer, Integer>  map = new HashMap<Integer, Integer> ();
        for(int i= 0; i< nums.length; i++){
            map.put(nums[i],i);
        }

        for(int i= 0; i< nums.length; i++){
            Integer v = map.get(target-nums[i]);
            if(v!=null && v>i){
                return new int []{i,v};
            }
        }
        return null;
    }
}