Algorithms

1. Count prime numbers not exceeding n(n>0)

2. Pseudocode:
   Input: an integer n > 1
    
   Let A be an array of Boolean values, indexed by integers 2 to n,
   initially all set to true.
    
    for i = 2, 3, 4, ..., not exceeding √n:
     if A[i] is true:
       for j = i2, i2+i, i2+2i, i2+3i, ..., not exceeding n :
         A[j] := false
    
   Output: all i such that A[i] is true.

   The Sieve of Eratosthenes uses an extra O(n) memory and its runtime complexity is O(n log log n).

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Algorithms

原文：http://www.cnblogs.com/wade-case/p/4567929.html