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.

    int min = Integer.MAX_VALUE;

    public int minDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        helper(root, 1);
        return min;
    }
    private void helper(TreeNode root, int depth) {   
        if (root.left == null && root.right == null) {
            min = Math.min(min, depth);            
        }
        if (root.left != null) {
            helper(root.left, depth + 1);
        }
        if (root.right != null) {
            helper(root.right, depth + 1);
        }
    }

或者：

   public static int minDepth(TreeNode root) {
    if (root == null)    return 0;
    if (root.left == null)    return minDepth(root.right) + 1;
    if (root.right == null) return minDepth(root.left) + 1;
    return Math.min(minDepth(root.left),minDepth(root.right)) + 1;
}

或者level order更好

求max depth:

   public int findHeight(TreeNode root) {
     if (root == null) {
       return 0;
     }
     return Math.max(findHeight(root.left), findHeight(root.right)) + 1;
   }