（1）Two Sum--求数组中相加为指定值的两个数

最新推荐文章于 2019-06-27 09:35:46 发布

转载最新推荐文章于 2019-06-27 09:35:46 发布 · 102 阅读

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原文链接：http://www.cnblogs.com/leehfly/p/4940310.html

本文详细解析了在整数数组中寻找两个数使它们的和等于特定目标数的算法。介绍了两种方法：一种是双层循环遍历，另一种是使用哈希表进行优化，仅需遍历一次数组。通过具体示例展示了算法的实现过程。

Given an array of integers, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution.

Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

answer:

 1 public class TwoSum {
 2     //最low的方法
 3     public int[] twoSum(int[] nums, int target) {
 4         int[] ret = new int[2];
 5         for (int i = 0; i < nums.length; i++) {
 6             for (int j = i + 1; j < nums.length; j++) {
 7                 if (nums[i] + nums[j] == target) {
 8                     ret[0] = i + 1;
 9                     ret[1] = j + 1;
10                 }
11             }
12         }
13         return ret;
14 
15     }
16 
17     // 利用map,只需要遍历一次即可
18     public int[] twoSum1(int[] numbers, int target) {
19         int[] ret = new int[2];    //初始化一个数组，默认值为0
20         Map<Integer, Integer> map = new HashMap<Integer, Integer>();
21         for (int i = 0; i < numbers.length; i++) {
22             if (map.containsKey(target - numbers[i])) {
23                 ret[1] = i + 1; //map中放的数字必然是比较早遍历过的
24                 ret[0] = map.get(target - numbers[i]);
25                 return ret;
26             }
27             map.put(numbers[i], i + 1);
28         }
29         return ret;
30     }
31 
32     //测试
33     public static void main(String[] args) {
34         int[] numbers = { 2, 7, 11, 15 };
35         int target = 9;
36         TwoSum test = new TwoSum();
37         int[] result = test.twoSum1(numbers, target);
38         System.out.println(Arrays.toString(result));
39     }
40 
41 }