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public class Solution {
    public int titleToNumber(String s) {
        if(s == null || s.length() == 0) return 0;
        int res = 0;
        for (int i = 0; i < s.length(); i++) {
            res = res * 26 + s.charAt(i) - 'A' + 1;
        }
        return res;
    }
}