1.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.

2.題目要求：求二叉樹(shù)的最小深度即最短路徑的節(jié)點(diǎn)個(gè)數(shù)。

3.方法：用遞歸的方法，深度優(yōu)先搜索DFS來(lái)完成。

4.代碼：
/**

• 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 minDepth(TreeNode *root) {
  if (root == NULL) return 0;
  if (root->left == NULL && root->right == NULL) return 1;

   if (root->left == NULL) return minDepth(root->right) + 1;
   else if (root->right == NULL) return minDepth(root->left) + 1;
   else return 1 + min(minDepth(root->left), minDepth(root->right));
  

  }

};