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.

//递归方法求二叉树的最小深度 //注意推断树的深度应该到叶子节点，也就是左右子结点都为空的那个结点 class Solution { public:	 int minDepth(TreeNode *root) {		 if (root == NULL) 			 return 0;		 if (root->left == NULL) 			 return minDepth(root->right) + 1;		 else if (root->right == NULL) 			 return minDepth(root->left) + 1;		 else 			 return min(minDepth(root->left), minDepth(root->right)) + 1;	 } };