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Question

Formatted question description: https://leetcode.ca/all/274.html

Given an array of integers citations where citations[i] is the number of citations a researcher received for their ith paper, return the researcher's h-index.

According to the definition of h-index on Wikipedia: The h-index is defined as the maximum value of h such that the given researcher has published at least h papers that have each been cited at least h times.

 

Example 1:

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, their h-index is 3.

Example 2:

Input: citations = [1,3,1]
Output: 1

 

Constraints:

• n == citations.length
• 1 <= n <= 5000
• 0 <= citations[i] <= 1000

Algorithm

1. Sort all SCI papers published by him in descending order of the number of citations;
2. Search the sorted list from front to back until the serial number of a paper is greater than the number of times the paper has been cited.

The obtained serial number minus one is the H index

Code

• Java
• C++
• Python
• Go
• TypeScript
• RenderScript

• 
  public class H_Index {
      class Solution {
          public int hIndex(int[] citations) {
  
              if (citations == null || citations.length == 0) {
                  return 0;
              }
  
              // @note
              Arrays.sort(citations); // 6,5,3,1,0
              for (int i = 0; i < citations.length / 2; ++i) {
                  int temp = citations[i];
                  citations[i] = citations[citations.length - i - 1];
                  citations[citations.length - i - 1] = temp;
              }
  
              for (int i = 0; i < citations.length; ++i) {
                  if (citations[i] <= i) { // @note: <i then return i+1 will not work
                      return i;
                  }
              }
  
              return citations.length;
  
          }
      }
  }
  
  
  ############
  
  class Solution {
      public int hIndex(int[] citations) {
          int n = citations.length;
          int[] cnt = new int[n + 1];
          for (int c : citations) {
              if (c <= n) {
                  ++cnt[c];
              } else {
                  ++cnt[n];
              }
          }
          int sum = 0;
          for (int i = n; i >= 0; --i) {
              sum += cnt[i];
              if (sum >= i) {
                  return i;
              }
          }
          return 0;
      }
  }
  
  

• // OJ: https://leetcode.com/problems/h-index/
  // Time: O(NlogN)
  // Space: O(1)
  class Solution {
  public:
      int hIndex(vector<int>& A) {
          sort(begin(A), end(A));
          int L = 0, R = A.size() - 1, N = A.size();
          while (L <= R) {
              int M = (L + R) / 2;
              if (A[M] < N - M) L = M + 1;
              else R = M - 1;
          }
          return N - L;
      }
  };
  

• class Solution:
      def hIndex(self, citations: List[int]) -> int:
          n = len(citations)
          cnt = [0] * (n + 1)
          for c in citations:
              if c <= n:
                  cnt[c] += 1
              else:
                  cnt[n] += 1
          sum = 0
          for i in range(n, -1, -1):
              sum += cnt[i]
              if sum >= i:
                  return i
          return 0
  
  ############
  
  class Solution(object):
    def hIndex(self, citations):
      """
      :type citations: List[int]
      :rtype: int
      """
      n = len(citations)
      dp = [0] * (n + 1)
      for c in citations:
        if c > n:
          dp[n] += 1
        else:
          dp[c] += 1
  
      total = 0
      for i in reversed(range(1, len(dp))):
        total += dp[i]
        if total >= i:
          return i
      return 0
  
  

• func hIndex(citations []int) int {
  	n := len(citations)
  	left, right := 0, n
  	for left+1 < right {
  		mid := int(uint(left+right) >> 1)
  		if check(citations, mid) {
  			left = mid
  		} else {
  			right = mid
  		}
  	}
  	if check(citations, right) {
  		return right
  	}
  	return left
  }
  
  func check(citations []int, mid int) bool {
  	cnt := 0
  	for _, citation := range citations {
  		if citation >= mid {
  			cnt++
  		}
  	}
  	return cnt >= mid
  }
  
  

• function hIndex(citations: number[]): number {
      let n = citations.length;
      let cnt = new Array(n + 1).fill(0);
      for (let c of citations) {
          if (c <= n) {
              ++cnt[c];
          } else {
              ++cnt[n];
          }
      }
      let sum = 0;
      for (let i = n; i > -1; --i) {
          sum += cnt[i];
          if (sum >= i) {
              return i;
          }
      }
      return 0;
  }
  
  

• impl Solution {
      #[allow(dead_code)]
      pub fn h_index(citations: Vec<i32>) -> i32 {
          let mut citations = citations;
          citations.sort_by(|&lhs, &rhs| {
              rhs.cmp(&lhs)
          });
  
          let n = citations.len();
  
          for i in (1..=n).rev() {
              if citations[i - 1] >= i as i32 {
                  return i as i32;
              }
          }
  
          0
      }
  }
  

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