力扣344题、541题、151题、剑指offer58-II（反转字符串，翻转字符串里的单词,左旋转字符串）

344、反转字符串

基本思想：

双指针中的左右指针

具体实现：

代码：

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;
        }
    }
}

541、反转字符串II

class Solution {
    public String reverseStr(String s, int k) {
        char[] ch = s.toCharArray();
        // 1. 每隔 2k 个字符的前 k 个字符进行反转
        for (int i = 0; i< ch.length; i += 2 * k) {
            // 2. 剩余字符小于 2k 但大于或等于 k 个，则反转前 k 个字符
            if (i + k <= ch.length) {
                reverse(ch, i, i + k -1);
                continue;
            }
            // 3. 剩余字符少于 k 个，则将剩余字符全部反转
            reverse(ch, i, ch.length - 1);
        }
        return  new String(ch);

    }
    // 定义翻转函数
    public void reverse(char[] ch, int i, int j) {
    for (; i < j; i++, j--) {
        char temp  = ch[i];
        ch[i] = ch[j];
        ch[j] = temp;
    }

    }
}

151、翻转字符串里的单词

具体实现：

1.移除多余空格

2.将整个字符串翻转

3.将每个单词翻转

举例：

• 移除多余空格 : "the sky is blue"
• 字符串反转："eulb si yks eht"
• 单词反转："blue is sky the"

class Solution {
    public String reverseWords(String s) {
        StringBuilder sb = removeSpace(s);
        reverseString(sb, 0, sb.length() - 1);
        reverseEachWorld(sb);
        return sb.toString();
    }

    private StringBuilder removeSpace(String s){
        int start = 0;
        int end = s.length() - 1;
        while (s.charAt(start) == ' ') start++;
        while (s.charAt(end) == ' ') end--;
        StringBuilder sb = new StringBuilder();
        while (start <= end){
            char c = s.charAt(start);
            if (c != ' ' || sb.charAt(sb.length() - 1) != ' '){//防止单词中间的空格有两个
                sb.append(c);
            }
            start++;
        }
        return sb;
    }

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

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

剑指offer58-II左旋转字符串

具体实现：

 代码：

class Solution {
    public String reverseLeftWords(String s, int n) {
        int len = s.length();
        StringBuilder sb = new StringBuilder(s);
        reverseString(sb, 0, n - 1);
        reverseString(sb, n, len - 1);
        return sb.reverse().toString();
    }
    public void reverseString(StringBuilder sb, int start, int end){
        while(start < end){
            char temp = sb.charAt(start);
            sb.setCharAt(start, sb.charAt(end));
            sb.setCharAt(end, temp);
            start++;
            end--;
        }
    }
}