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package class32;
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public class Problem_0204_CountPrimes {
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public static int countPrimes(int n) {
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if (n < 3) {
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return 0;
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}
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// j已经不是素数了，f[j] = true;
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boolean[] f = new boolean[n];
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int count = n / 2; // 所有偶数都不要，还剩几个数
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// 跳过了1、2 3、5、7、
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for (int i = 3; i * i < n; i += 2) {
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if (f[i]) {
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continue;
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}
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// 3 -> 3 * 3 = 9 3 * 5 = 15 3 * 7 = 21
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// 7 -> 7 * 7 = 49 7 * 9 = 63
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// 13 -> 13 * 13 13 * 15
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for (int j = i * i; j < n; j += 2 * i) {
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if (!f[j]) {
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--count;
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f[j] = true;
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}
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}
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}
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return count;
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}
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}
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