代码随想录二刷day08

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前言

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一、力扣344. 反转字符串

class Solution {
    public void reverseString(char[] s) {
        for(int i = 0, j = s.length-1; i < j; i++, j --){
            char temp = s[i];
            s[i] = s[j];
            s[j] = temp;
        }
    }
}

二、力扣541. 反转字符串 II

class Solution {
    public String reverseStr(String s, int k) {
        char[] ch = s.toCharArray();
        int low, high;
        for(low = 0, high = k-1; high < ch.length;low += 2*k, high += 2*k){
            reverseFun(ch, low,high);
        }
        if(low < ch.length){
            reverseFun(ch, low, ch.length-1);
        }
        return new String(ch);
    }
    public void reverseFun(char[] ch, int low, int high){
        for(int i = low, j = high; i < j; i ++, j --){
            char temp = ch[i];
            ch[i] = ch[j];
            ch[j] = temp;
        }
    }
}

三、力扣剑指 Offer 05. 替换空格

class Solution {
    public String replaceSpace(String s) {
        // List list = new ArrayList<>();
        StringBuilder sb = new StringBuilder();
        for(int i = 0;i < s.length(); i ++){
            if(s.charAt(i) == ' '){
                sb.append('%');
                sb.append('2');
                sb.append('0');
            }else{
                sb.append(s.charAt(i));
            }
        }
        return sb.toString();
    }
}

双指针法

class Solution {
    public String replaceSpace(String s) {
        int count = 0;
        for(int i = 0; i < s.length(); i ++){
            if(s.charAt(i) == ' '){
                count++;
            }
        }
        if(count == 0){
            return s;
        }
        int len = s.length() + 2*count;
        char[] res = new char[len];
        for(int i = s.length()-1, j = len-1; i >= 0;){
            if(s.charAt(i) == ' '){
                i--;
                res[j--] = '0';
                res[j--] = '2';
                res[j--] = '%';
            }else{
                res[j--] = s.charAt(i--);
            }
        }
        return new String(res);
    }
}

四、力扣151. 反转字符串中的单词

class Solution {
    public String reverseWords(String s) {
        StringBuilder sb = new StringBuilder();
        int low, high = 0,i , j;
        for(j = 0; j < s.length();){
            while(j < s.length() && s.charAt(j) == ' ')j++;
            if(j == s.length()){
                break;
            }
            i = j;
            while(j < s.length() && s.charAt(j) != ' ')j++;
            low = i; high = j-1;
            while(high >= low){
                sb.append(s.charAt(high--));
            }
            sb.append(' ');
        }
        sb.deleteCharAt(sb.length()-1);
        sb.reverse();
        return sb.toString();
    }
}

五、力扣剑指 Offer 58 - II. 左旋转字符串

class Solution {
    public String reverseLeftWords(String s, int n) {
        char[] ch = s.toCharArray();
        fun(ch, 0, n-1);
        fun(ch, n, s.length()-1);
        fun(ch, 0, s.length()-1);
        return String.valueOf(ch);
    }
    public void fun(char[] ch, int low, int high){
        for(int i = low, j = high; i < j; i ++, j --){
            char temp = ch[i];
            ch[i] = ch[j];
            ch[j] = temp;
        }
    }
}