gistfile1.java

class Solution{
  public static boolean isPermutation(String s1, String s2){
    if(s1 === null || s2 == null ) return false;
    if(s1.length() != s2.length()) return false;
    if(s1.length() == 0) return true; // If s1.length() == s2.length() and s1 is a empty string, s2 is the permutation of s1.
    
    char[] sc1 = s1.toCharArray();
    char[] sc2 = s2.toCharArray();
    
    HashMap<String, int> temp = new HashMap<String, boolean>();
    
    for(char c: sc1) {
      if(!temp.containsKey(c))
        temp.put(c, 1);
      else{
        int count = temp.get(c) +1;
        temp.put(c, count);
      };
    }

    for(char c: sc2) {
      if(!temp.containsKey(c))
        return false;
      else{
        int count = temp.get(c) - 1;
        if(count<0) return false;
        else temp.put(c, count);
      };
    }
    
    return true;
    
  }
}

I guess you didn't test your code...