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/**

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

*

* For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

*

* 1

* / \

* 2 2

* / \ / \

* 3 4 4 3

*

* But the following [1,2,2,null,3,null,3] is not:

* 1

* / \

* 2 2

* \ \

* 3 3

*

* Note:

* Bonus points if you could solve it both recursively and iteratively.

*

*/

/**

* Definition for a binary tree node.

* public class TreeNode {

* int val;

* TreeNode left;

* TreeNode right;

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

* }

*/

public class SymmetricTree101 {

public boolean isSymmetric(TreeNode root) {

if (root == null || (root.left == null && root.right == null)) return true;

LinkedList<TreeNode> queue = new LinkedList<>();

queue.add(root.left);

queue.add(root.right);

int i = 1;

while (!queue.isEmpty()) {

Stack<TreeNode> st = new Stack<>();

i = queue.size() >> 1;

int k = 0;

while (k<i) {

if (queue.isEmpty()) return false;

TreeNode node = queue.remove();

if (node != null) {

queue.add(node.left);

queue.add(node.right);

}

st.push(node);

k++;

}

k = 0;

while (k<i) {

if (queue.isEmpty()) return false;

TreeNode node = queue.remove();

TreeNode pre = st.pop();

if (node != null) {

if (pre == null || node.val != pre.val) return false;

queue.add(node.left);

queue.add(node.right);

} else if (pre != null) {

return false;

}

k++;

}

}

return true;

}

/**

* https://discuss.leetcode.com/topic/5941/recursive-and-non-recursive-solutions-in-java

*/

public boolean isSymmetric2(TreeNode root) {

if (root==null) return true;

Stack<TreeNode> stack = new Stack<TreeNode>();

TreeNode left, right;

if (root.left != null) {

if (root.right == null) return false;

stack.push(root.left);

stack.push(root.right);

} else if (root.right != null) {

return false;

}

while (!stack.empty()) {

if (stack.size()%2 != 0) return false;

right = stack.pop();

left = stack.pop();

if(right.val != left.val) return false;

if (left.left != null) {

if(right.right == null) return false;

stack.push(left.left);

stack.push(right.right);

} else if(right.right != null){

return false;

}

if (left.right != null) {

if(right.left == null) return false;

stack.push(left.right);

stack.push(right.left);

} else if(right.left != null) {

return false;

}

}

return true;

}

/**

* https://discuss.leetcode.com/topic/5941/recursive-and-non-recursive-solutions-in-java

*/

public boolean isSymmetric3(TreeNode root) {

return root==null || isSymmetricHelp(root.left, root.right);

}

private boolean isSymmetricHelp(TreeNode left, TreeNode right){

if(left==null || right==null)

return left==right;

if(left.val!=right.val)

return false;

return isSymmetricHelp(left.left, right.right) && isSymmetricHelp(left.right, right.left);

}

}