Arrays_Strings 翻转字符串 @CareerCup

原文：

Write code to reverse a C-Style String. (C-String means that “abcd” is represented as five characters, including the null character.)

译文：

写代码翻转一个C风格的字符串。(C风格的意思是"abcd"需要用5个字符来表示，包含末尾的 结束字符)

写了多个字符串翻转的方法：

package Arrays_Strings;

import java.util.Stack;

public class S1_2 {

	// 首尾位置交换
	public static String reverse1(String s) {
		char[] cc = s.toCharArray();
		int len = s.length();
		for(int i=0; i<len/2; i++) {
			char tmp = cc[i];
			cc[i] = cc[len-1-i];
			cc[len-1-i] = tmp;
		}
		return new String(cc);
	}
	
	// Java库函数
	public static String reverse2(String s) {
		return new StringBuilder(s).reverse().toString();
	}
	
	// 反向添加
	public static String reverse3(String s) {
		String ret = "";
		for(int i=s.length()-1; i>=0; i--) {
			ret += s.charAt(i);
		}
		return ret;
	}
	
	// 递归1
	public static String reverse4(String s) {
		StringBuilder ret = new StringBuilder();
		rec(new StringBuilder(s), ret, 0);
		return ret.toString();
	}
	
	// 不能用String作为param，否则无法改变String的值，建议一律采用StringBuilder
	public static void rec(StringBuilder src, StringBuilder dest, int pos) {
		if(pos == src.length()) {
			return;
		}
		rec(src, dest, pos+1);
		dest.append(src.charAt(pos));
	}
	
	// 递归2
	public static String reverse5(String s) {
		if(s.length() <= 1) {
			return s;
		}
		
		return reverse5(s.substring(1)) + s.charAt(0); 
	}
	
	
	// =========================================
	// 输入“I am a student.”，则输出“student. a am I”
	public static String reverseSentence1(String s) {
		// 翻转“I am a student.”中所有字符得到“.tneduts a ma I”，
		// 再翻转每个单词中字符的顺序得到“students. a am I”
		String reversedSentence = reverse1(s);
		String[] ss = reversedSentence.split(" ");
		String ret = "";
		for(int i=0; i<ss.length; i++) {
			ret += reverse1(ss[i]);
			if(i != ss.length-1) {
				ret += " ";
			}
		}
		return ret;
	}
	
	// 用一个辅助栈
	public static String reverseSentence2(String s) {
		Stack<Character> stack = new Stack<Character>();
		String ret = "";
		
		//  先将整句话倒着进栈
		for(int i=s.length()-1; i>=0; i--) {
			char c = s.charAt(i);
			if(c == ' '){		// 进栈前检查是否为" ",如果为TRUE,先将依次POP()
				while( !stack.isEmpty() ) {
					ret += stack.pop();
				}
				ret += " ";
			} else{
				stack.push(c);
			}
		}
		
		// 栈内可能还有剩下的字符
		while( !stack.isEmpty() ) {
			ret += stack.pop();
		}
		
		return ret;
	}
	
	
	
	
	public static void main(String[] args) {
		String s = "abc";
		System.out.println(reverse1(s));
		System.out.println(reverse2(s));
		System.out.println(reverse3(s));
		System.out.println(reverse4(s));
		System.out.println(reverse5(s));
		
		s = "I am a student.";
		System.out.println(reverseSentence1(s));
		System.out.println(reverseSentence2(s));
	}
}