Symmetric Tree.java

Symmetric TreeSep 24 '12
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
Note:
Bonus points if you could solve it both recursively and iteratively.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSymmetric(TreeNode root) {
        ArrayList<String> inOrder = new ArrayList<String>();
        inOrder(root, inOrder);
        int start = 0, end = inOrder.size() - 1;
        while (start < end) {
            if (inOrder.get(start).equals(inOrder.get(end))) {
                start++;
            	end--;
            } else {
            	return false;
            }
        }
        return true;
    }
    
    public void inOrder(TreeNode root, ArrayList<String> inOrder) {
        if (root == null) {
            inOrder.add("#");
            return;
        }
        inOrder(root.left, inOrder);
        inOrder.add(root.val + "");
        inOrder(root.right, inOrder);
    }
}