Smallest substring of all characters - mock interview - review peer code, very good practice

SmallestSubstringOfAllCharacters_MockInterview.cs

import java.io.*;
import java.util.*;

class Solution {  
  /*
    left=0 right=0
    min
    x, y, y, z
       y . y . z y zyx
                   zyx
  */
  static String getShortestUniqueSubstring(char[] arr, String str) {
    // your code goes here
    if(arr == null || arr.length == 0 || str == null || str.length() == 0){
      return "";
    }
    
    int[] count = new int[256];
    
    for(char ch : arr){
      count[ch]++;
    }
    
    int cnt = arr.length;
    
    int left = 0;
    int right = 0;
    
    int min = Integer.MAX_VALUE;
    int minLeft  = -1;
    int minRight = -1;
    
    while(right < str.length()){
      char ch = str.charAt(right);
      
      count[ch]--;
      
      if(count[ch] >= 0){
        cnt--;
      }
      
      while(cnt <= 0){
        if(min > right-left+1){
          min = right-left+1;
          minLeft = left;
          minRight = right;
        }
        
        char prev = str.charAt(left);
        
        count[prev]++;
        
        if(count[prev] > 0){
          cnt++;  // break the loop, prev is in unique char array
        }
        
        left++;
      }
      
      right++;
    }
    
    if(minLeft == -1 && minRight == -1){
      return "";
    }
    
    return str.substring(minLeft,minRight+1);
  }
  
  public static void main(String[] args) {
  }
}