String Permutation

Given two strings, write a method to decide if one is a permutation of the other.

Example

abcd is a permutation of bcad, but abbe is not a permutation of abe

 public class Solution {
     /**
      * @param A a string
      * @param B a string
      * @return a boolean
      */
     public boolean stringPermutation(String A, String B) {
         // Write your code here
         if(A==null&&B==null){
             return true;
         } else if (A==null||B==null||A.length()!=B.length()){
             return false;
         }
         Map<Character, Integer> a = new HashMap<Character, Integer>();

         for(char c : A.toCharArray()){
             if(!a.containsKey(c))
                 a.put(c, 1);
             else
                 a.put(c, a.get(c)+1);

             //a.put(c, a.getOrDefault(c, 0) + 1);
         }
         for(char c : B.toCharArray()){
             if(!a.containsKey(c) || a.get(c)==0){
                 return false;
             }
             a.put(c, a.get(c)-1);
         }
         return true;
     }
 }