02-统计字符串字符

/**
 *  给定字符串，统计每个字符出现的次数
 */
public class Test15 {
	public static void main(String[] args) {
		String s = "1239586838923173478943890234092";
		
		long nanos = System.nanoTime();
		countChars1(s); // for+for 逐个计数、查重
		System.out.println("\n for+for运行时间（纳秒）：" +  (System.nanoTime()-nanos)); // 约79w纳秒

		System.out.println("-------------------------");
		nanos = System.nanoTime();
		countChars2(s); // replace() 重复的替换为空，两个字符串长度相减
		System.out.println("\n replace运行时间（纳秒）：" +  (System.nanoTime()-nanos)); // 约250w纳秒

		System.out.println("-------------------------");
		nanos = System.nanoTime();
		countChars3(s); // new int[10] 使用0-9下标作为数字，元素作计数
		System.out.println("\n int[10]运行时间（纳秒）：" +  (System.nanoTime()-nanos)); // 约39w纳秒
		
		System.out.println("-------------------------");
		nanos = System.nanoTime();
		countChars4(s); // regex 正则匹配直接替换出相同的字符组成字符串，长度即数量
		System.out.println("\n regex运行时间（纳秒）：" +  (System.nanoTime()-nanos)); // 约50w~80w纳秒不稳定

	}
	
	public static void countChars1(String s) {
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < s.length(); i++) {
			int count = 0;
			Character ch = new Character(s.charAt(i));
			if (sb.indexOf(ch.toString()) >= 0) { // 利用StringBuilder的indexOf()方法查重
				continue;
			}

			for (int j = 0; j < s.length(); j++) {
				if (ch == s.charAt(j)) {
					sb.append(ch);
					count++;
				}
			}
			System.out.println(ch + "出现次数：" + count);
		}
	}
	
	public static void countChars2(String s) {
		for (int i = 0; i < 10; i++) {
			String str = s.replace(String.valueOf(i), ""); // 将1-9单个相同的数字替换出来，剩下的保存
			System.out.println(i + "出现次数：" + (s.length()-str.length()));
		}
	}
	
	public static void countChars3(String s) {
		int[] c = new int[10];
		
		for(int i = 0 ; i < s.length(); i++){
			char ch = s.charAt(i);
			int a = ch-48;
			c[a]++;
		}
		
		for (int i = 0; i < c.length; i++) {
			System.out.println(i + "出现次数：" + c[i]);
		}
	}
	
	public static void countChars4(String s) {
		for (int i = 0; i < 10; i++) {
			String str2 = s.replace("[^"+i+"]", "");
			System.out.println(i + "出现次数：" + str2.length());
		}
	}
}