https://leetcode.com/problems/minimum-depth-of-binary-tree/description/

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.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7]

    3
   / \
  9  20
    /  \
   15   7

return its minimum depth = 2.

題解：注意左右子樹是否非空

class Solution {
public:
    int minDepth(TreeNode* root) {
      if (root == NULL){
        return 0;
      }
      while (root != NULL){
        if (root->left == NULL && root->right != NULL){
          return minDepth(root->right) + 1;
        }
        if (root->left != NULL && root->right == NULL){
          return minDepth(root->left) + 1;
        }
        return min(minDepth(root->left), minDepth(root->right)) + 1;
      }
    }
};