Count Primes
Description:
Count the number of prime numbers less than a non-negative number, n.
Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.
我們考慮1-n的數 我們只需要考慮他們是否為素數 可以用bitset做
對於一個數,我們可以一次性將它的倍數設置為true
這樣false位的數就是素數
public int countPrimes(int n) { BitSet bs = newBitSet(n); int count = 0; for(int i=2;i<n;i++) { if(!bs.get(i)) { count ++; for(int j=2;j*i<n;j++) bs.set(j*i); } } return count; }
Count Primes
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