[LeetCode][JavaScript]Count Primes

Count Prime

Description:

Count the number of prime numbers less than a non-negative number, n.

https://leetcode.com/problems/count-primes/

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找出所有小于n的数中的质数。

删数法。开一个1到n的数组，删除所有2的倍数，3的倍数...直到√n的倍数，最后剩下的就是质数。

/**
 * @param {number} n
 * @return {number}
 */
var countPrimes = function(n) {
    var count = 0, i, j, dict = [],
        len = Math.floor(Math.sqrt(n));
    for(i = 2; i <= len; i++)
        for(j = 2; i * j <= n; j++)
            dict[i * j] = false;
    for(i = 2; i < n; i++)
        if(dict[i] === undefined) count++;
    return count;
};