【leetcode】【1】Two Sum

一、问题描述

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].

二、问题分析

比较经典的一位数组的处理问题。可以利空HashMap来处理，key存nums[i]和target的差值，value存数组的索引。时间复杂度O（n）。

三、Java AC 代码

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