[LeetCode] 101. Symmetric Tree

Given the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center).

Example 1:

Input: root = [1,2,2,3,4,4,3]
Output: true

Example 2:

Input: root = [1,2,2,null,3,null,3]
Output: false

Constraints:

• The number of nodes in the tree is in the range [1, 1000].
• -100 <= Node.val <= 100

所谓的对称，就是每一个node的左子树都等于它的右子树。做法和之前的same tree的问题很类似，只不过这次是一个node的左子树和另一个node的右子树比较是否相同。第一次调用这个function的时候，两个node都是root。

/**
 * Definition for a binary tree node.
 * 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 isSymmetric(TreeNode root) {
        return isSame(root, root);
    }
    
    public boolean isSame(TreeNode p, TreeNode q) {
        if (p == null ^ q == null) {
            return false;
        }
        if (p == null && q == null) {
            return true;
        }
        return p.val == q.val && isSame(p.left, q.right) && isSame(p.right, q.left);
    }
}