204. Count Primes - LeetCode

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204. Count Primes

Solution

題目大意：給一個數，求小於這個數的素數的個數

思路：初始化一個boolean數組，初始設置爲true,先遍歷將2的倍數設置爲false，再遍歷3並將3的倍數置爲false...

Java實現：

此法超時

public int countPrimes(int n) {
    // 2, 3, 5, 7
    boolean[] candidate = new boolean[n];
    Arrays.fill(candidate, Boolean.TRUE);
    candidate[0] = false;
    candidate[1] = false;
    int count = 0;
    for (int i=2; i<n; i++) {
        if (candidate[i]) {
            count++;
            for (int j = i+1; j<n; j++) {
                if (j%i==0) candidate[j] = false;
            }
        }
    }
    return count;
}

參考別人的：

public int countPrimes(int n) {
    boolean[] notPrime = new boolean[n];
    int count = 0;
    for (int i = 2; i < n; i++) {
        if (notPrime[i] == false) {
            count++;
            for (int j = 2; i*j < n; j++) {
                notPrime[i*j] = true;
            }
        }
    }
    return count;
}