100. Same Tree (Tree;DFS)

 Given two binary trees, write a function to check if they are equal or not.

Two binary trees are considered equal if they are structurally identical and the nodes have the same value.

思路：如果两个树相同，那么他们的左子树、右子树必定也相同=>递归=>可用前序、中序或后续遍历。

以下用的是前序遍历，先处理根节点。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSameTree(TreeNode* p, TreeNode* q) {
        if (p==NULL && q==NULL)  return true;
        else if(p!=NULL && q!=NULL)
        {  
            if(p->val != q->val) return false;
        }
        else return false;

        
        if (!isSameTree(p->left,q->left)) return false;
        
        if (!isSameTree(p->right,q->right)) return false;
        
        return true;
    }
};