字串的左旋轉

package su.interview;/** * 定義字串的左旋轉操作：把字串前面的若干個字元移動到字串的尾部。如把字串abcdef左旋轉2位得到字串cdefab。請實現字串左旋轉的函數。 *  * @author Toy *  */public class String_06 {/** * 分成兩部分拼接 O(n)+O(n) *  * @param str * @param step * @return */public String rotate_01(String str, int step) {char[] c = str.toCharArray();StringBuffer s = new StringBuffer();for (int i = step; i < str.length(); i++) {s.append(c[i]);}for (int i = 0; i < step; i++) {s.append(c[i]);}str = s.toString();return str;}/** * 三次旋轉 O(n)+O(1) *  * @param str * @param step * @return */public String rotate_02(String str, int step) {char[] c = str.toCharArray();reverse(c, 0, step);reverse(c, step, c.length);reverse(c, 0, c.length);str = String.valueOf(c);return str;}private void reverse(char[] c, int i, int step) {char temp = '0';int j = step - 1;while (i < j) {temp = c[i];c[i] = c[j];c[j] = temp;i++;j--;}}/** * 每次迴圈移動一位 O(n*k)+O(1) *  * @param str * @param step * @return */public String rotate_03(String str, int step) {step=step%str.length();//在step很大的時候可以縮小規模char[] c = str.toCharArray();while (step > 0) {char temp = c[0];for (int i = 1; i < c.length - 1; i++) {c[i - 1] = c[i];}c[c.length-1]=temp;step--;}str = String.valueOf(c);return str;}/** * @param args */public static void main(String[] args) {String_06 s = new String_06();String str = "abcdefghijk";System.out.println(str);str = s.rotate_01(str, 5);System.out.println(str);str = s.rotate_02(str, 5);System.out.println(str);str = s.rotate_03(str, 5);System.out.println(str);}}