Problem 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.

Solution:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
/**
* Definition for a binary tree node.
* 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) return 0; //NULL node no height
        if(!root->left) return 1+minDepth(root->right); //edge case for no left subtree
        if(!root->right) return 1+minDepth(root->left);
        
        return 1+min(minDepth(root->left), minDepth(root->right));
    }
};

reference:
https://goo.gl/mNYNmX