Problem #101: Symmetric Tree

Intuition & Strategy Breakdown

Use helper recursive function to check if left subtree is a mirror reflection of right subtree (left.val == right.val, left.left == right.right, left.right == right.left).

Python Solution

class Solution:
    def isSymmetric(self, root: Optional[TreeNode]) -> bool:
        if not root:
            return True
        
        def isMirror(t1, t2):
            if not t1 and not t2:
                return True
            if not t1 or not t2:
                return False
            return (t1.val == t2.val) and \
                   isMirror(t1.left, t2.right) and \
                   isMirror(t1.right, t2.left)
        
        return isMirror(root.left, root.right)

Java Solution

class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        return isMirror(root.left, root.right);
    }

    private boolean isMirror(TreeNode t1, TreeNode t2) {
        if (t1 == null && t2 == null) return true;
        if (t1 == null || t2 == null) return false;
        return (t1.val == t2.val) 
            && isMirror(t1.left, t2.right) 
            && isMirror(t1.right, t2.left);
    }
}