111. Minimum Depth of Binary Tree

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.

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

无法像maxdepth那里一步到位，因为要判断是不是空的。