代码随想录训练营第八天｜ 344.反转字符串，541. 反转字符串II ，剑指Offer 05.替换空格，151.翻转字符串里的单词，剑指Offer58-II.左旋转字符串

第八天，从昨天看，难度应该会递加，今天是五道题，稍有挑战性了。

class Solution {
    public void reverseString(char[] s) {
        int n = s.length;
        int i = 0;
        int j = n - 1;

        while(i<j){
            
            char temp = s[i];
            s[i] = s[j];
            s[j] = temp;
            i++;
            j--;

        }
    }
}

很简单双指针首尾互换即可

541. 反转字符串2

class Solution {
    public String reverseStr(String s, int k) {
        int n = s.length();
        StringBuffer sb = new StringBuffer();
        int l = 0;
        while(l < n){
            StringBuffer temp = new StringBuffer();
            int count = (l + k > n) ? n : l + k;
            int jump = (l + 2*k >n ) ? n : l + 2*k;
            temp.append(s.substring(l, count));
            sb.append(temp.reverse());

            if(count < jump){
                sb.append(s.substring(count, jump));

            }
            l += 2*k;
        }
        return sb.toString();
    }
}

string中的char，相隔k个反转k个，如果结尾剩余的不够的话，全部反转

剑指offer 05. 替换空格

class Solution {
    public String replaceSpace(String s) {
        int n = s.length();
        StringBuilder sb = new StringBuilder();
        for(int i = 0; i<n; i++){
            if(s.charAt(i)!=' '){
                sb.append(s.charAt(i));
            }
            else{
                sb.append("%20");
            }
        }
        return sb.toString();
    }
}

直接替换

151. 反转字符串里的单词

class Solution {
    public String reverseWords(String s) {
        
        char[] s_char = s.toCharArray();
        int slow = 0;
        int fast = 0;
        
        reverseArray(s_char, 0, s_char.length-1);
        
        
        reverseWord(s_char);

        return cleanSpace(s_char);
        
        // return new String(s_char).substring(0, s_char.length);
    }
    public void reverseArray(char[] array, int i, int j ){
        while(i < j){
            char temp = array[i];
            array[i++] = array[j];
            array[j--] = temp;
        }
    }
    public void reverseWord(char[] array){
        int n = array.length;
        int i = 0;
        int j = 0;
        while (j< n ){
            while( i < n && array[i] == ' '){
                i++;

            }
            j = i;
            while(j < n && array[j] != ' '){
                j++;
            }
            reverseArray(array, i, j-1);
            i = j;
        }
        
    }
    public String cleanSpace(char[] array){
        int n = array.length;
        int i = 0;
        int j = 0;
        while (j < n) {
            while (j < n && array[j] == ' ') j++;
            while (j < n && array[j] != ' ') array[i++] = array[j++];
            while (j < n && array[j] == ' ') j++;
            if (j < n) array[i++] = ' ';
        }
        return new String(array).substring(0, i);
        
    }
        
    
}

先反转数组，再反转单词，再清除多余空格

剑指offer58 左旋字符串2

class Solution {
    public String reverseLeftWords(String s, int n) {
        int size = s.length();

        StringBuilder sb = new StringBuilder();
        for(int i = n; i<size; i++){
            sb.append(s.charAt(i));
        }
        for(int i = 0; i< n; i++){
            sb.append(s.charAt(i));
        }
        return sb.toString();
    }
}