TreeNode.java

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {
    }

    TreeNode(int val) {
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

class Solution {
    public boolean isSameTree(TreeNode p, TreeNode q) {

        if (p == null && q == null) {
            return true;
        }
        if (p == null || q == null || p.val != q.val) {
            return false;
        }
        return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);

    }

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

    private boolean isHelper(TreeNode left, TreeNode right) {
        if (left == null && right == null) {
            return true;
        }
        if (left == null || right == null) {
            return false;
        }

        return (left.val == right.val) && isHelper(left.left, right.right) && isHelper(left.right, right.left);
    }

}