树：面试题55

面试题55 - I. 二叉树的深度

解题思路：通过层次遍历，将每一层只算一次，进行while循环弹出该层所有数据，在进行下一层入队列。

/**
 * 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:
    int maxDepth(TreeNode* root) 
    {
        int   lev=0;
        queue<TreeNode* > q;
        if (root == NULL)
            return 0;
        q.push(root);
        while (!q.empty() )
        {
            ++lev;
            int len = q.size();
            while(len--)
            {
                TreeNode* node = q.front();
                q.pop();
                if (node->left != NULL)
                    q.push(node->left);
                if (node->right != NULL)
                    q.push(node->right);          
            }
        }
        return lev;

    }
};

解题思路2：递归算法，求得树的最大深度，要设定好结束条件。

/**
 * 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:
    int maxDepth(TreeNode* root) 
    {
        if (root == NULL)
            return 0;
        return max(maxDepth(root->left), maxDepth(root->right) ) + 1; 
    }
};