[LeetCode]101. Symmetric Tree

Easy

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

1ms:

 public boolean isSymmetric(TreeNode root) {
        if(root==null)
            return true;
        return isSym(root.left,root.right);
    }
    private boolean isSym(TreeNode n1, TreeNode n2){
        if(n1==null&&n2==null)
            return true;
        else if((n1==null&&n2!=null)||(n1!=null&&n2==null))
            return false;


        if(n1.val==n2.val&&isSym(n1.left,n2.right)&&isSym(n1.right,n2.left))
            return true;
        else
            return false;
    }