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111. 二叉树的最小深度

给定一个二叉树,找出其最小深度。
最小深度是从根节点到最近叶子节点的最短路径上的节点数量。
说明:叶子节点是指没有子节点的节点。

示例 1:

 3
/ \
9 20
  / \
 15 7

输入:root = [3,9,20,null,null,15,7]
输出:2

示例 2:
输入:root = [2,null,3,null,4,null,5,null,6]
输出:5

提示:
树中节点数的范围在 [0, 105] 内
-1000 <= Node.val <= 1000

javascript
/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var minDepth = function (root) {
  if (!root) return 0
  if (root.left && !root.right) {
    return minDepth(root.left) + 1
  } else if (!root.left && root.right) {
    return minDepth(root.right) + 1
  }
  return Math.min(minDepth(root.left), minDepth(root.right)) + 1
}

// test
function TreeNode(val, left, right) {
  this.val = val === undefined ? 0 : val
  this.left = left === undefined ? null : left
  this.right = right === undefined ? null : right
}

var n1 = new TreeNode(2)
var n2 = new TreeNode(3)
var n3 = new TreeNode(4)
var n4 = new TreeNode(5)
var n5 = new TreeNode(6)
n1.right = n2
n2.right = n3
n3.right = n4
n4.right = n5

console.log(minDepth(n1))