Leetcode 01 Two Sum

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于 2016-09-24 22:50:19 发布

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本文链接：https://blog.youkuaiyun.com/Hermann_weyl/article/details/52654444

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本文介绍了一种经典的算法问题——寻找数组中两个数相加等于特定目标值的索引。提供了两种解决方案：一种是通过双重循环进行暴力匹配；另一种是使用HashMap提高查找效率。

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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");
}