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;
    }
}