Solution.java

class Solution {
    public String reformat(String s) {
        StringBuilder a = new StringBuilder();
        StringBuilder b = new StringBuilder();
        for (char c : s.toCharArray()) {
            if (Character.isDigit(c)) {
                a.append(c);
            } else {
                b.append(c);
            }
        }
        int m = a.length(), n = b.length();
        if (Math.abs(m - n) > 1) {
            return "";
        }
        StringBuilder ans = new StringBuilder();
        for (int i = 0; i < Math.min(m, n); ++i) {
            if (m > n) {
                ans.append(a.charAt(i));
                ans.append(b.charAt(i));
            } else {
                ans.append(b.charAt(i));
                ans.append(a.charAt(i));
            }
        }
        if (m > n) {
            ans.append(a.charAt(m - 1));
        }
        if (m < n) {
            ans.append(b.charAt(n - 1));
        }
        return ans.toString();
    }
}