面試題16：遞歸計算二叉樹的最大、最小深度

用遞歸方法計算二叉樹的最大、最小深度，注意他們之間的區別

• Given a binary tree, find its minimum depth.The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.
• Given a binary tree, find its maximum depth.The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

class Solution {
public:

    int run(TreeNode *root) {
        if(root == nullptr) return 0;
        if(root->left == nullptr) {
            return run(root->right) + 1;
        }
        if(root->right == nullptr){
            return run(root->left) + 1;
        }
        int left = run(root->left);
        int right = run(root->right);
        return min(left,right)+1;
    }
};

/**
 * Definition for binary tree
 * 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 res = 0;
        maxDepth(root, res);
        return res;
    }

    int maxDepth(TreeNode* root, int& depth) {
        if(root == nullptr) return 0;

        int left = maxDepth(root->left,depth);
        int right = maxDepth(root->right,depth);
        int d = max(left,right) + 1;
        if(d > depth){
            depth = d;
        }
        return d;
    }
};