Symmetric Tree

LeetCode #101

Description:

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

Example:

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

Idea:

bool isSymmetric(TreeNode* root) parameter只有一个root，不好写，但把parameter变为两个root，就很好写了。

Note，不用改变function number，通过不同的parameter可以overloading。

Code:

class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if(root==NULL) return true;

        return isSymmetric(root->left, root->right);
    }

    bool isSymmetric(TreeNode *p, TreeNode *q){
        if(p==NULL && q==NULL) return true;
        if(p==NULL || q==NULL) return false;
        if(p->val!=q->val) return false;

        return isSymmetric(p->left, q->right)&&isSymmetric(p->right, q->left);
    }
};