static int excelNum(String header){
        int sum = 0;
        if(header != null){
            int len = header.length(); 
            for(int i=0; i<len; i++){
                int n = header.charAt(len-1 - i) - 'A' + 1;
                sum += n*Math.pow(26, i);
            }
        }
        return sum;
    }

// 0 -> 'A'
    // 1 -> 'B'
    // 25 -> 'Z'
    // num = 1 -> 'A'
    // num = 26 -> 'A' + 25 = 'Z'
    // num = 27
    // reminder = 0 -> 'A'
    // num = 1
    // reminder = 0 -> 'A'
    // num = 52
    // (52 - 1) % 26 = 25 -> 'Z'
    // 52 / 26 = 2
    //  x = ['A' - 'Z'] - 'A'
    //  x = [0 ... 25]
    //  
    //  n = x + 1
    //  sum = (x + 1)*pow(26, 2) + (x + 1)*pow(26, 1) + (x+1)*pow(26, 0)
    static String numToHeader(int num){
        String str = "";
        while(num > 0){
            int reminder = (num - 1) % 26;
            char ch = (char)(reminder + 'A');
            str = ch + "" + str;
            num = (num - 1) / 26;
        }
        return str;
    }