代码随想录Day_08|Leetcode.344.541.卡码网54.151（StringBuilder可变字符串，问题分解看，Character.isDigit，charAt）

344.翻转字符串

双指针

class Solution {
    public void reverseString(char[] s) {
        int fir=0;
        int sec=s.length-1;
        char temp='\0';
        while(fir<sec){
            temp=s[fir];
            s[fir]=s[sec];
            s[sec]=temp;
            fir++;
            sec--;
        }
    }
}

XOR（异或）翻转

s[l] ^= s[r];  
s[r] ^= s[l];  
s[l] ^= s[r];

541.翻转字符串Ⅱ

class Solution {
    public String reverseStr(String s, int k) {
        char[] N = s.toCharArray();
        for (int i = 0; i < N.length - 1; i += 2 * k) {
            if (i + 2 * k - 1 <= N.length - 1) {
                reverseString(N, i, i + k - 1);
            } else if (i + k - 1 <= N.length - 1) {
                reverseString(N, i, i + k - 1);
            } else {
                reverseString(N, i, N.length - 1);
            }
        }
        return new String(N);
    }


    public void reverseString(char[] s, int fir, int sec) {
        char temp = '\0';
        while (fir < sec) {
            temp = s[fir];
            s[fir] = s[sec];
            s[sec] = temp;
            fir++;
            sec--;
        }
    }
}

卡码网54. 替换数字

.nextLine获取字符串，.nextInt获取整数

StringBuilder创建可变长字符串 StringBuffer创建可变长线程安全数组，但是效率低

Character.isDigit()判断是否为数字，.charAt（）获取字符串对应索引位置的char字符

import java.util.Scanner;

class Main{
    public static void main (String[] args) {
        Scanner scanner = new Scanner(System.in);
        String s = scanner.nextLine();
        StringBuilder res = new StringBuilder();
        for (int i = 0;i < s.length() ; i++ ){
            if(Character.isDigit(s.charAt(i))){
                res.append("number");
            }else{
                res.append(s.charAt(i));
            }
        }
        System.out.println(res);

    }
}

151.翻转字符串中的单词

class Solution {
    public String reverseWords(String s) {
        StringBuilder res = removeSpace(s);
        //去除多余空格
        reverseString(res, 0, res.length() - 1);
        //翻转整个字符串
        reverseEachWord(res);
//翻转每一个单词
        return res.toString();
    }

    public StringBuilder removeSpace(String res) {
        int start = 0;
        int end = res.length() - 1;
        while (res.charAt(start) == ' ') start++;
        while (res.charAt(end) == ' ') end--;
        StringBuilder sb = new StringBuilder();
        for (; start <= end; start++) {
            if (res.charAt(start) != ' ' || sb.charAt(sb.length() - 1) != ' ') {
                sb.append(res.charAt(start));
            }

        }
        return sb;
    }

    public void reverseString(StringBuilder res, int start, int end) {
        while (start < end) {
            char temp = res.charAt(start);
            res.setCharAt(start, res.charAt(end));
            res.setCharAt(end, temp);
            start++;
            end--;

        }
    }

    private void reverseEachWord(StringBuilder res) {
        int start = 0;
        int end = 1;
        while (start < res.length()) {
            while (end < res.length() && res.charAt(end) != ' ') {
                end++;
            }
            reverseString(res, start, end - 1);
            start = end + 1;
            end = start + 1;
        }
    }
}

卡码网.右旋字符串

import java.util.Scanner;

class Main{
    public static void main(String []args){
        Scanner scan=new Scanner(System.in);
        int k=Integer.parseInt(scan.nextLine());
        String str=scan.nextLine();
        
        int len=str.length();
        char[] chars=str.toCharArray();
        reverseString(chars,0,len-1);
        reverseString(chars,0,k-1);
        reverseString(chars,k,len-1);
        
        System.out.println(chars);

    }
    
    public static void reverseString(char[] ch,int start,int end){
        while(start<end){
            ch[start]^=ch[end];
            ch[end]^=ch[start];
            ch[start]^=ch[end];
            start++;
            end--;
        }
    }
    
}