JZ-043-Left rotation string

Rotate string left

Title description

There is a shift instruction in assembly language called Rotate Left (ROL). Now there is a simple task, which is to simulate the operation result of this instruction with a string. For a given character sequence S,

• Please output the sequence after it is circularly shifted K bits to the left. For example, the character sequence S="abcXYZdef", it is required to output the result of the circular shift left by 3 bits, that is, "XYZdefabc". Is not it simple? OK, get it done!

title link : left rotation string

Code

/**
 * 标题：左旋转字符串
 * 题目描述
 * 汇编语言中有一种移位指令叫做循环左移（ROL），现在有个简单的任务，就是用字符串模拟这个指令的运算结果。对于一个给定的字符序列S，
 * 请你把其循环左移K位后的序列输出。例如，字符序列S=”abcXYZdef”,要求输出循环左移3位后的结果，即“XYZdefabc”。是不是很简单？OK，搞定它！
 * 题目链接：
 * https://www.nowcoder.com/practice/12d959b108cb42b1ab72cef4d36af5ec?tpId=13&&tqId=11196&rp=1&ru=/ta/coding-interviews&qru=/ta/coding-interviews/question-ranking
 */
public class Jz43 {

    /**
     * 方法一：用额外的空间
     *
     * @param str
     * @param n
     * @return
     */
    public String leftRotateString(String str, int n) {
        if (str == null || str.length() == 0) {
            return str;
        }
        char[] result = new char[str.length()];
        for (int i = 0; i < str.length(); i++) {
            int newIndex = (i - n + str.length()) % str.length();
            result[newIndex] = str.charAt(i);
        }
        return new String(result);
    }

    /**
     * 方法二：字符串操作
     *
     * @param str
     * @param n
     * @return
     */
    public String leftRotateString2(String str, int n) {
        if (n >= str.length()) {
            return str;
        }
        char[] chars = str.toCharArray();
        reverse(chars, 0, n - 1);
        reverse(chars, n, chars.length - 1);
        reverse(chars, 0, chars.length - 1);
        return new String(chars);
    }

    private void reverse(char[] chars, int i, int j) {
        while (i < j) {
            swap(chars, i++, j--);
        }
    }

    private void swap(char[] chars, int i, int j) {
        char t = chars[i];
        chars[i] = chars[j];
        chars[j] = t;
    }

    public static void main(String[] args) {

    }
}

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