Tree Minimum depth

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.

Definition of a tree:

// Java
class TreeNode
{
public:
  int val;
  TreeNode left;
  TreeNode right;
  TreeNode(int x) {
    val = x;
    left = NULL; 
    right = NULL;
};

// C++
class TreeNode
{
public:
  int val;
  TreeNode *left;
  TreeNode *right;
  TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

Implement function minDepth that returns the minimum depth of the tree.

// Java
int minDepth(TreeNode tree)

// C++
int minDepth(TreeNode *tree)

Example

Function minDepth returns 2 because the shortest path from the root 5 to the nearest leaf 10 contains 2 nodes.