1.Two Sum

bakergus

于 2016-11-12 23:59:49 发布

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分类专栏： Leetcode Easy Array HashTable 文章标签： Hash-Table Array

本文链接：https://blog.youkuaiyun.com/bakergus/article/details/53144913

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本文介绍了一种利用哈希表解决LeetCode上经典问题“两数之和”的方法。通过创建哈希表存储数组元素及其索引，算法能在O(n)时间内找出两个数的索引，使它们相加等于目标值。

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Click here to try this problem on Leetcode

Problems with tag: Array
Problems with tag : Hash Table
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].

UPDATE (2016/2/13):
The return format had been changed to zero-based indices. Please read the above updated description carefully.

思路：这道题目比较简单，直接建立Hash Table来解决就可以了。方法是，先建立一个nums数组的Hash Table；然后for-loop依次遍历每个元素，看是否能够在Hash Table中找到target - nums[i]，如果能够找到，就返回i和target - nums[i]在Hash Table中对应的下标。

C++代码如下：

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
    vector<int> res;
    unordered_map<int,int> map;

    for(int i = 0; i < nums.size(); i++){
        int gap = target - nums[i];
        if(map.find(gap) != map.end())
            res = {i, map[gap]};
        else
            map[nums[i]] = i;
    }
    return res;
    }
};