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，找到质数，排除其所有的倍数
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排除后剩下的数据
var ans = 0;
for (var p = 2; p < n; p++) {
if (!exclude[p]) {
ans++;
}
}

return ans;
};