# 274. H-Index (1)

[Link](https://leetcode.com/problems/h-index/description/)

**Description**

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](https://en.wikipedia.org/wiki/H-index): "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."

**Example:**

```
Input: citations = [3,0,6,1,5]
Output: 3 
Explanation: [3,0,6,1,5] 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, her h-index is 3.
```

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

**Solution**

h值是paper 數，且滿足至少有h篇paper的citation數大於h

```
class Solution {
    public int hIndex(int[] citations) {
            int n = citations.length;
            //Collect the count of each citation number into the buckets
            // and the index is the citation number, the value is the count
            // for ex: bucket[0] = 1, means there are one paper which citation count is 0
            // for ex: bucket[2] = 3, means there are 3 papers which citation count is 2
            int[] buckets = new int[n+1];
            for(int i = 0; i < n; i++){
                if(citations[i] > n)
                {
                    buckets[n]++;
                }
                else{
                    buckets[citations[i]]++;
                }
            }
        
            //iterate the buckets from back to front
            //and calucalate the paperCount for each iteration
            int count = 0;
            //count表示citation是i以上的paper數
            for(int i = n; i >= 0; i--) {
                count += buckets[i];
                if(count >= i) {
                    return i;
                }
            }
            return 0;
    }
}
```
