101. Symmetric Tree.md

101. Symmetric Tree

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

Follow up

Solve it both recursively and iteratively.

---

Solution

• mine

  • Java

    Recursive Runtime: 0 ms, faster than 100.00%, Memory Usage: 37.3 MB, less than 74.15% of Java online submissions

    //O(N)time O(1)space
    public boolean isSymmetric(TreeNode root) {
        return check(root,root);
    }
    
    public boolean check(TreeNode left,TreeNode right){
        if(left == null && right == null){
            return true;
        }
        if(left == null || right == null){
            return false;
        }
        return left.val == right.val && check(left.left,right.right) && check(left.right,right.left); 
    }