Minimum Depth of Binary Tree

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.

Solution: 1. Recursion. Pay attention to cases when the non-leaf node has only one child.
2. Iteration + Queue.

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     int minDepth(TreeNode *root) {
13         if(!root) return 0;
14         if(!root->left && !root->right) return 1;
15         if(!root->left) {
16             return 1 + minDepth(root->right);
17         }
18         if(!root->right) {
19             return 1 + minDepth(root->left);
20         }
21         return 1 + min(minDepth(root->left), minDepth(root->right));
22     }
23 };