408. Valid Word Abbreviation

class Solution {
    public boolean validWordAbbreviation(String word, String abbr) {
        if (word == null && abbr == null) return true;
        if (word == null || abbr == null) return false;
        
        int i = 0;
        int j = 0;
        while (i < word.length() && j < abbr.length()) {
            if (word.charAt(i) == abbr.charAt(j)) {
                i++;
                j++;
            } else if (abbr.charAt(j) > '0' && abbr.charAt(j) <= '9') { // Corner case: If the number starts with '0', it's invalid.
                int start = j;
                while (j < abbr.length() && abbr.charAt(j) >= '0' && abbr.charAt(j) <= '9') {
                    j++;
                }
                int num = Integer.valueOf(abbr.substring(start, j));
                i += num;
            } else {
                return false;
            }
        }
        return (i == word.length()) && (j == abbr.length());
    }
}