Symmetric Tree

SymmetricTree.java

/*
Symmetric Tree
Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3
But the following is not:
    1
   / \
  2   2
   \   \
   3    3
*/

import java.util.Stack;

public class Solution{
    public boolean isSymmetric(TreeNode root) {
        if (root == null)
            return true;

        Stack<TreeNode> s1 = new Stack<TreeNode>();
        Stack<TreeNode> s2 = new Stack<TreeNode>();
        s1.add(root.left);
        s2.add(root.right);
        while (!s1.isEmpty() || !s2.isEmpty()) {
            TreeNode a = s1.pop();
            TreeNode b = s2.pop();
            if (a == null && b == null)
                continue;
            if (a == null || b == null)
                return false;
            if (a.val != b.val)
                return false;

            s1.add(a.left);
            s1.add(a.right);

            s2.add(b.right);
            s2.add(b.left);
        }

        return true;

    }

    class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int x) {
            val = x;
        }
    }

}