代码随想录算法训练营DAY8

代码随想录算法训练营DAY8 | 字符串(1)

一、LeetCode 344 反转字符串

题目链接： 344.反转字符串https://leetcode.cn/problems/reverse-string/

思路：双指针法交换。

class Solution {
    public void reverseString(char[] s) {
        int n = s.length;
        int left = 0, right = n-1;
        while(left < right){
            char temp = s[left];
            s[left] = s[right];
            s[right] = temp;
            left++;
            right--;
        }
    }
}

二、LeetCode 541 反转字符串II

题目链接：541.反转字符串IIhttps://leetcode.cn/problems/reverse-string-ii/

思路：双指针按块反转。

class Solution {
    public String reverseStr(String s, int k) {
        char[] cs = s.toCharArray();
        int n = cs.length;
        int left = 0, right = 0;
        while(n >= 2*k){
            right = left+k-1;
            reverse(cs,left,right);
            n -= 2*k;
            left = left + 2*k;
        }
        if(n >= k){
            right = left+k-1;
            reverse(cs,left,right);
        }else{
            reverse(cs,left,cs.length-1);
        }
        return new String(cs);
    }
    public void reverse(char[] cs, int left, int right){
        while(left < right){
            char temp = cs[left];
            cs[left] = cs[right];
            cs[right] = temp;
            left++;
            right--;
        }
    }
}

三、卡码网 54 替换数字

题目链接：54.替换数字https://kamacoder.com/problempage.php?pid=1064

思路：使用StringBuilder类的append方法，遍历字符串得到答案。

import java.util.*;
import java.lang.*;
public class Main{
    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        StringBuilder sb = new StringBuilder();
        for(int i = 0; i < s.length(); i++){
            if(s.charAt(i) >= '0' && s.charAt(i) <= '9'){
                sb.append("number");
            }else{
                sb.append(s.charAt(i));
            }
        }
        System.out.print(sb.toString());
    }
}

四、LeetCode 151 反转字符串中的单词

题目链接：151.反转字符串中的单词https://leetcode.cn/problems/reverse-words-in-a-string/

思路：先去除首尾空格，再把字符串中的单词都存储到ArrayList中，最后倒序输出到Stringbuilder中，得到最终结果。

class Solution {
    public String reverseWords(String s) {
        List<String> list = new ArrayList<>();
        String ss = s.trim();
        char[] cs = ss.toCharArray();
        int left = 0;
        for(int i = 0; i < cs.length; i++){
            if(cs[i] == ' '){
                list.add(ss.substring(left,i));
                while(cs[i] == ' '){
                    i++;
                }
                left = i;
            }
        }
        list.add(ss.substring(left,cs.length));
        StringBuilder sb = new StringBuilder();
        for(int i = list.size()-1; i >= 0; i--){
            sb.append(list.get(i));
            sb.append(" ");
        }
        return sb.toString().trim();
    }
}

五、卡码网 55 右旋转字符串

题目链接： 55.右旋转字符串

思路：先让k进行对n的求余，再截取后k个字符为子串1，剩余字符为子串2，创建StringBuilder把子串1和子串2合成可得出答案。

import java.util.*;
import java.lang.*;
public class Main{
    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        int k = sc.nextInt();
        String s = sc.next();
        int n = s.length();
        k = k%n;
        String s1 = s.substring(n-k,n);
        String s2 = s.substring(0,n-k);
        StringBuilder sb = new StringBuilder();
        sb.append(s1);
        sb.append(s2);
        System.out.print(sb.toString());
    }
}