274. H-Index

题目

Given an array of citations (each citation is a non-negative integer) of a researcher, write a function to compute the researcher's h-index.

According to the ​​definition of h-index on Wikipedia​​: "A scientist has index h if h of his/her N papers have at least h citations each, and the other N − h papers have no more than h

For example, given ​​citations = [3, 0, 6, 1, 5]​​​, which means the researcher has ​​5​​​ papers in total and each of them had received ​​3, 0, 6, 1, 5​​​ citations respectively. Since the researcher has ​​3​​ papers with at least ​​3​​ citations each and the remaining two with no more than ​​3​​​ citations each, his h-index is ​​3​​.

Note: If there are several possible values for ​​h​​, the maximum one is taken as the h-index.

思路

本题思路比较简单，但是容易理解错误题意，这题目的意思是给定一个数组，数组大小为N，表示一位研究人员一共发了N篇文章，其中数组中的数值代表每篇文章被引用的次数。我们所要求解的h大小需要满足条件：

A scientist has index h if h of his/her N papers have at least h citations each, and the other N − h papers have no more than h citations each.

本题采用一个hashtable来存储每个hash表中数字出现的次数，其中数字大于N的统一处理（见代码）。下面给一个特殊例子，利于大家理解：

[6,7,8,9,10]，这里h=5：当数组里的数值均大于N=5时，h=N

代码

class Solution {
public:
    int hIndex(vector<int>& citations) {
        int size = citations.size(),res=0;
        if(size<=0)
            return 0;
        unordered_map<int,int> mp;
        for(auto item: citations){
            if(item>size)
                mp[size]++;
            else
                mp[item]++;
        }
        for(int i=size;i>=0;i--)
        {
            res+=mp[i];
            if(res>=i)
                return i;
        }
    }
};