Leetcode: Valid Word Abbreviation

Given a non-empty string s and an abbreviation abbr, return whether the string matches with the given abbreviation.

A string such as "word" contains only the following valid abbreviations:

["word", "1ord", "w1rd", "wo1d", "wor1", "2rd", "w2d", "wo2", "1o1d", "1or1", "w1r1", "1o2", "2r1", "3d", "w3", "4"]
Notice that only the above abbreviations are valid abbreviations of the string "word". Any other string is not a valid abbreviation of "word".

Note:
Assume s contains only lowercase letters and abbr contains only lowercase letters and digits.

Example 1:
Given s = "internationalization", abbr = "i12iz4n":

Return true.
Example 2:
Given s = "apple", abbr = "a2e":

Return false.

public class Solution {
    public boolean validWordAbbreviation(String word, String abbr) {
        int i = 0, j = 0;
        while (i<word.length() && j<abbr.length()) {
            if (!isDigit(abbr.charAt(j))) {
                if (word.charAt(i) != abbr.charAt(j)) return false;
                i++;
                j++;
            }
            else {
                int num = 0;
                while (j<abbr.length() && isDigit(abbr.charAt(j))) {
                    num = num*10 + (int)(abbr.charAt(j)-'0');
                    if (num == 0) return false; //"001" with '0' at front should return false
                    j++;
                }
                i = i + num;
            }
        }
        if (i==word.length() && j==abbr.length()) return true;
        return false;
    }
    
    public boolean isDigit(char c) {
        if (c>='0' && c<='9') return true;
        return false;
    }
}