Count Primes - Javascript

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.

Tags

Hash Table, Math

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/**
 * @param {number} n
 * @return {number}
 */
var countPrimes = function(n) {
    // Prime Numbers within 10 
    // 2 3 5 7 9
    if(n <= 1) {
        return 0;
    }
    
    // 排除法，找到質數，排除其所有的倍數
    var exclude = [];
    for(var i=2;i<n;i++) {
        var k=2;
        while( i*k < n) {
            if(!exclude[i*k]) {
                exclude[i*k] = true;
            }
            k++;
        }
    }
    
    // 搜索1~n排除後剩下的數據
    var ans = 0;
    for (var p = 2; p < n; p++) {
        if (!exclude[p]) {
            ans++;
        }
    }

    return ans;
};