101. Symmetric Tree

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

Version #2 Iteration
一个很神的Level order traversal
每次还是按Mirror的顺序往queue里面加TreeNode就好了
12.65 %

public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        Queue<TreeNode> que = new LinkedList<>();
        que.offer(root.left);
        que.offer(root.right);
     
        TreeNode leftNode;
        TreeNode rightNode;
        while (que.size() > 1) {
            leftNode = que.poll();
            rightNode = que.poll();
            if (leftNode == null && rightNode == null) continue;
            if (leftNode == null || rightNode == null || leftNode.val != rightNode.val) return false;
            que.offer(leftNode.left);
            que.offer(rightNode.right);
            que.offer(leftNode.right);
            que.offer(rightNode.left);
        }
        return true;
    }
}



Version #1 Recursion

二刷
22.55 %
public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        return isSymmetric(root.left, root.right);
    }
    private boolean isSymmetric(TreeNode leftRoot, TreeNode rightRoot) {
        if (leftRoot == null && rightRoot == null) return true;
        if (leftRoot == null || rightRoot == null) return false;
        if (leftRoot.val != rightRoot.val) return false;
        return isSymmetric(leftRoot.left, rightRoot.right) && isSymmetric(leftRoot.right, rightRoot.left);
    }
 
}



一刷
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        return isSymmetric(root.left, root.right);
    }
    private boolean isSymmetric(TreeNode left, TreeNode right) {
        if (left == null && right == null) {
            return true;
        } else if (left == null || right == null) {
            return false;
        } else if (left.val != right.val) {
            return false;
        } else {
            //System.out.println(left.val + " " + right.val);
            return isSymmetric(left.left, right.right) && isSymmetric(left.right, right.left);
        }
    }
}