leetcode 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.

让找出二叉树的最小深度

思路：
深度的定义是从root到叶子节点的最短路径，注意一定是到叶子节点

比如root只有左子树，没有右子树，这时候不能说最短深度是1，因为root本身不是叶子节点，所以必须是到左子树深度+1

所以当左或右子树为null时，要看另一边子树的深度+1
都不是null时，选最小深度

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