leetcode_344

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1题目：给一个字符串，反向输出：
例如：“hello”，输出“olleh”

2代码：
package com.leetcode;

import java.util.Scanner;
import java.util.Stack;

public class C344 {

public static void main(String args[]) {    Scanner in = new Scanner(System.in);    System.out.println("please input String:");    String str = in.next();    System.out.println(reverse6(str));}public static String reverse1(String s) {    int length = s.length();    if(length <= 1) {        return s;    }    String left = s.substring(0,length/2);    String right = s.substring(length/2,length);    return reverse1(right) + reverse1(left);}public static String reverse2(String s) {    char[] chars = new char[s.length()];    int index = 0;    for(int i = s.length() - 1;i>=0;i--) {        chars[index++] = s.charAt(i);    }    return new String(chars);}public static String reverse3(String s) {    String reverse = "";    for(int i = s.length() -1;i>=0;i--) {        reverse += s.charAt(i);    }    return reverse;}public static String reverse4(String s) {    return new StringBuffer(s).reverse().toString();}public static String reverse5(String s) {    char[] chars = s.toCharArray();    int n = chars.length-1;    for(int i=0;i<=n/2;i++) {        char tem = chars[i];        chars[i] = chars[n-i];        chars[n-i] = tem;    }    return new String(chars);}public static String reverse6(String s) {    char[] chars = s.toCharArray();    int begin = 0;    int end = chars.length - 1;    while (begin < end ) {        chars[begin] = (char) (chars[begin] ^ chars[end]);        chars[end] = (char) (chars[begin] ^ chars[end]);        chars[begin] = (char) (chars[begin] ^ chars[end]);        begin++;        end--;    }    return new String(chars);}public static String reverse7(String s){    char[] chars = s.toCharArray();    Stack<Character> stack = new Stack<>();    String reverse = "";    for(int i = 0;i<chars.length;i++) {        stack.push(chars[i]);    }    for(int i = 0;i<chars.length;i++) {        reverse += stack.pop();    }    return reverse;}

}

3分析： java解决这个问题方法很容易想到多种，以上就是我列出来的7种方法。
但大体可以分为三类：利用java已有的封装好的方法、利用交换、利用堆栈。
（1）利用java已有的封装好的方法
public static String reverse1(String s) {//用二分法，递归
int length = s.length();
if(length <= 1) {
return s;
}
String left = s.substring(0,length/2);
String right = s.substring(length/2,length);

    return reverse1(right) + reverse1(left);}public static String reverse2(String s) {    char[] chars = new char[s.length()];    int index = 0;    for(int i = s.length() - 1;i>=0;i--) {        chars[index++] = s.charAt(i);    }    return new String(chars);}public static String reverse3(String s) {    String reverse = "";    for(int i = s.length() -1;i>=0;i--) {        reverse += s.charAt(i);    }    return reverse;}public static String reverse4(String s) {    return new StringBuffer(s).reverse().toString();}

（2）利用交换
public static String reverse5(String s) {
char[] chars = s.toCharArray();
int n = chars.length-1;

    for(int i=0;i<=n/2;i++) {        char tem = chars[i];        chars[i] = chars[n-i];        chars[n-i] = tem;    }    return new String(chars);}public static String reverse6(String s) {//效率会高一点，不费空间实现交换    char[] chars = s.toCharArray();    int begin = 0;    int end = chars.length - 1;    while (begin < end ) {        chars[begin] = (char) (chars[begin] ^ chars[end]);        chars[end] = (char) (chars[begin] ^ chars[end]);        chars[begin] = (char) (chars[begin] ^ chars[end]);        begin++;        end--;    }    return new String(chars);}

（3）利用堆栈

public static String reverse7(String s){        char[] chars = s.toCharArray();        Stack<Character> stack = new Stack<>();        String reverse = "";        for(int i = 0;i<chars.length;i++) {            stack.push(chars[i]);        }        for(int i = 0;i<chars.length;i++) {            reverse += stack.pop();        }        return reverse;    }

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