Leetcode 01 Two Sum

题目：Given an array of integers, return indices(索引) 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].

方法1：暴力解法

public class Solution{    public int[] twoSum(int[] nums, int target)     {          int[] a = new int[2];        for (int i = 0; i < nums.length; i++)       {           for (int j = i + 1; j < nums.length; j++)           if  ( (nums[i] + nums[j]) == target)           {               a[0] = i;               a[1] = j;           }       }       return a;    }}

方法2：利用 HashMap

public int[] twoSum(int[] nums, int target) {    Map<Integer, Integer> map = new HashMap<>();    for (int i = 0; i < nums.length; i++) {        int complement = target - nums[i];        if (map.containsKey(complement)) {            return new int[] { map.get(complement), i };        }        map.put(nums[i], i);    }    throw new IllegalArgumentException("No two sum solution");}