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 citations each."

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.

Hint:

An easy approach is to sort the array first.

What are the possible values of h-index?

A faster approach is to use extra space.

链接： http://leetcode.com/problems/h-index/

题解：

求H-Index，找在数组中有k个元素大于等于k。排序一下再进行计算就比较容易些了。 也可以用index-couting，但这样就需要一个额外的数组。

Time Complexity - O(nlogn)， Space Complexity - O(1)。

public class Solution {
public int hIndex(int[] citations) {
if(citations == null || citations.length == 0) {
return 0;
}
int len = citations.length;
for(int i = 0; i < len; i++) {
if(citations[i] >= len - i) {    // check if we have len - i elements larger than len - i
return len - i;
}
}
return 0;
}
}

Index counting:

Time Complexity - O(n)， Space Complexity - O(n)

public class Solution {
public int hIndex(int[] citations) {
if(citations == null || citations.length == 0) {
return 0;
}
int len = citations.length;
int[] count = new int[len + 1];
for(int num : citations) {
if(num > len) {
count[len]++;
} else {
count[num]++;
}
}

int sum = 0;
for(int i = len; i >= 0; i--) {
sum += count[i];
if(sum >= i) {
return i;
}
}

return 0;
}
}

Reference:
https://leetcode.com/discuss/55958/my-easy-solution https://leetcode.com/discuss/56041/a-clean-o-n-solution-in-java https://leetcode.com/discuss/55952/my-o-n-time-solution-use-java https://leetcode.com/discuss/55950/1-line-ruby-5-lines-c-6-lines-java https://leetcode.com/discuss/66656/java-o-n-time-with-easy-explanation