Question:

Given a binary tree, find its maximum depth.
The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Solution:

public int maxDepth(TreeNode root) { 
  if (root == null) return 0;       
  Deque<TreeNode> stack = new LinkedList<TreeNode>(); 
  stack.push(root); 
  int count = 0; 
  while (!stack.isEmpty()) { 
    int size = stack.size(); 
    while (size-- > 0) {            
      TreeNode cur = stack.pop(); 
      if (cur.left != null) 
        stack.addLast(cur.left); 
      if (cur.right != null) 
        stack.addLast(cur.right);
      }
    count++;    
   } 
  return count;
}