左旋转字符串

package com.String;

//旋转字符串
/** 
 * Q 26 左旋转字符串 
 * 题目：定义字符串的左旋转操作：把字符串前面的若干个字符移动到字符串的尾部。 
 * 如把字符串abcdef左旋转2位得到字符串cdefab。 
 * 请实现字符串左旋转的函数。要求时间对长度为n的字符串操作的复杂度为O(n)，辅助内存为O(1)。 
 */  
public class TestString04 {
	public static void main(String args[]) {
		String str = "abcdefg";
		int n = 3;
		fun1(str, n);
		System.out.println();
		String re = leftRotateString(str, 3);  
        System.out.println(re); 
	}

	public static void fun1(String str, int n) {
		char[] c = str.toCharArray();
		char[] c1 = new char[str.length()];
		int l = n + 1;
		for (int i = 0; i < n; i++) {
			c1[l] = c[i];
			l++;
		}
		int len = c.length - n - 1;
		for (int m = 0; m < str.length() - n; m++) {
			c1[m] = c[len];
			len++;

		}

		for (int i = 0; i < str.length(); i++) {
			System.out.print(c1[i]);
		}

	}

	public static String leftRotateString(String str, int n) {
		if (str == null || str.length() == 0) {  
            return str;  
        }  
        if (n <= 0 || n >= str.length()) {  
            return str;  
        }  
        int begin = 0;  
        int end = str.length() - 1;  
        char[] letters = str.toCharArray();  
        reverseString(letters, begin, n - 1);  
        reverseString(letters, n, end);  
        reverseString(letters, begin, end);  
        return new String(letters);  
	}
    public static String reverseString(char[] letters, int begin, int end) {        
        if (begin >= end) {  
            return null;  
        }  
        for (int i = begin, j = end; i < j; i++, j--) {  
            char tmp = letters[i];  
            letters[i] = letters[j];  
            letters[j] = tmp;  
        }  
        return new String(letters);  
    }  
}