CCI 9.5 字符串全排列

编写一个方法，确定某字符串的所有排列组合。

package cci;

import java.util.ArrayList;

public class CCI_9_5 {
	
	public static ArrayList<String> permutation(String str){
		if(str==null)
			return null;
		ArrayList<String> result = new ArrayList<String>();
		if(str.length()<=1){
			result.add(str);
			return result;
		}
		char first = str.charAt(0);
		String sub = str.substring(1);
		ArrayList<String> perms = permutation(sub);
		for(String perm : perms){
			for(int i=0; i<=perm.length(); i++){
				String temp = insertCharAt(perm, i, first);
				result.add(temp);
			}
		}
		return result;
			
	}
	private static String insertCharAt(String str, int index, char c){
		String pre = str.substring(0,index);
		String post = str.substring(index);
		return pre+c+post;
	}
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		String str = "abc";
		ArrayList<String> result = permutation(str);
		for(String item : result){
			System.out.println(item);
		}

	}

}