class Solution { public String convert(String s) { StringBuilder ans = new StringBuilder(s.length()); convert(s, ans, 0); return ans.toString(); } private int convert(String preorder, StringBuilder inorder, int start) { if (preorder.charAt(start) == '#') { return 1; } int offset = convert(preorder, inorder, start + 1); inorder.append(preorder.charAt(start)); return offset + convert(preorder, inorder, start + offset + 1) + 1; } }