Java实现字符串的全排列

package com.leetcode;

import java.util.ArrayList;

public class Permutation {

public static void main(String[] args) {
ArrayList<String> res = perms2("abc");
System.out.println(res);
}

//法一：
public static ArrayList<String> perms1(String s){
ArrayList<String> res = new ArrayList<String>();
if(s == null)
return null;
if(s.isEmpty()){
res.add("");
return res;
}
for(int i = 0; i < s.length(); i++){
char c = s.charAt(i);
String start = s.substring(0, i);
String end = s.substring(i + 1);
ArrayList<String> words = perms1(start + end);
for(String word : words){
String newStr = c + word;
res.add(newStr);
}
}
return res;
}

//法二：
public static ArrayList<String> perms2(String s){
ArrayList<String> res = new ArrayList<String>();
if(s == null)
return null;
if(s.isEmpty()){
res.add("");
return res;
}
char c = s.charAt(0);
String reminder = s.substring(1);
ArrayList<String> words = perms2(reminder);
for(String word : words){
for(int i = 0; i <= word.length(); i++){
String newStr = insertCharAt(word, c, i);
res.add(newStr);
}
}
return res;
}

public static String insertCharAt(String s, char c, int i){
String start = s.substring(0, i);
String end = s.substring(i);
return start + c + end;
}
}