Convert a Ternary expression to a Binary tree structure

Use stack to keep track of parent nodes.

#include <iostream>
#include <string>
#include <stack>
using namespace std;

struct TreeNode {
  char val;
  TreeNode *left;
  TreeNode *right;
  TreeNode(char c): val(c), left(NULL), right(NULL) {}
};

// To execute C++, please define "int main()"
TreeNode *CovertTt2Bt(string input)
{
  int n = input.size();
  if (!n) return NULL;

  TreeNode *res = new TreeNode(input[0]);
  TreeNode *cur = res;
  stack<TreeNode *> s;
  s.push(res);
  
  for (int i = 1; i < n; i = i + 2) {
    if (input[i] == '?') {
      TreeNode *t = new TreeNode(input[i+1]);
      cur->left = t;
      cur = t;
      s.push(cur);
    } else if (input[i] == ':') {
      do {
        s.pop();
      }
      while (s.top()->right);
      TreeNode *t = new TreeNode(input[i+1]);
      s.top()->right = t;
      cur = t;
    } 
  }
  
  return res;
}

void printTree(TreeNode *root) {
  if (!root) return;
  printf("%c ", root->val);
  printTree(root->left);
  printTree(root->right);
}

int main() {
  string s("a?b?c:d:e");
  TreeNode *res = CovertTt2Bt(s);
  printTree(res);
  return 0;
}