T27.java

package web; /**
 * @program LeetNiu
 * @description: 字符串的排列
 * @author: mf
 * @create: 2020/01/13 21:52
 */


import java.util.ArrayList;
import java.util.TreeSet;

/**
 * 输入一个字符串,按字典序打印出该字符串中字符的所有排列。
 * 例如输入字符串abc,则打印出由字符a,b,c所能排列出来的所有字符串abc,acb,bac,bca,cab和cba
 */
public class T27 {
    public ArrayList<String> Permutation(String str) {
        ArrayList<String> result = new ArrayList<>();
        if(str == null || str.length() == 0) {
            return result;
        }
        char[] chars = str.toCharArray();
        TreeSet<String> temp = new TreeSet<>();
        Permutation(chars, 0, temp);
        result.addAll(temp);
        return result;
    }

    private void Permutation(char[] chars, int begin, TreeSet<String> result) {
        if (chars == null || chars.length == 0 || begin < 0 || begin > chars.length - 1) {
            return;
        }
        if (begin == chars.length - 1) {
            result.add(String.valueOf(chars));
        } else {
            for (int i = begin; i <= chars.length - 1; i++) {
                swap(chars, begin, i);
                Permutation(chars, begin + 1, result);
                swap(chars, begin, i);
            }
        }
    }

    /**
     * 交换
     * @param x
     * @param a
     * @param b
     */
    private void swap(char[] x, int a, int b) {
        char t = x[a];
        x[a] = x[b];
        x[b] = t;
    }
}