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

思路：两条dfs

public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root==null) return true;

        return dfs(root.left,root.right);
    }

    public boolean dfs(TreeNode left, TreeNode right){
        if (left==null && right==null){
            return true;
        }else if(left==null || right==null){
            return  false;
        }

        if (left.val!=right.val){
            return false;
        }

        if (dfs(left.left,right.right)&&dfs(left.right,right.left)){
            return true;
        }

        return false;
    }
}

另一种code写法

public class Solution{
    public boolean isSymmetric(TreeNode root){
        return symmetricHelper(root,root);
    }

    public boolean symmetricHelper(TreeNode left, TreeNode right){
        if(left == null && right == null){
            return true;
        }else if(left != null && right != null && left.val == right. val){
            return symmetricHelper(left.left, right.right) && symmetricHelper(left.right, right.left);
        }
        return false;
    }
}