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.

注意，左边为空的时候，只返回右边的+1，右边为空的时候，只返回左边的+1，只有两边都不为空的时候，才返回两者的最小值+1；

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int minDepth(TreeNode root) {
        if(root == null) return 0;
        if(root.left == null && root.right == null) return 1;
        int leftdepth = minDepth(root.left);
        int rightdepth = minDepth(root.right);
        if(leftdepth == 0) return rightdepth+1;
        if(rightdepth == 0) return leftdepth+1;
        return Math.min(leftdepth, rightdepth)+1;
    }
}