给定二叉树 [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
返回它的最小深度  2.

let minDepth = function (root) {
  let min = Infinity
  let helper = (node, depth) => {
    if (!node) return
    if (!node.left && !node.right) {
      min = Math.min(min, depth)
      return
    }
    if (node.left) {
      helper(node.left, depth + 1)
    }
    if (node.right) {
      helper(node.right, depth + 1)
    }
  }
  helper(root, 1)

  return min === Infinity ? 0 : min
}

let minDepth = function (root) {
  if (!root) return 0

  let depth = 0
  let queue = [root]

  while (queue.length) {
    depth++
    let len = queue.length
    while (len--) {
      let node = queue.shift()

      let left = node.left
      let right = node.right
      if (!left && !right) {
        return depth
      }

      if (left) {
        queue.push(left)
      }
      if (right) {
        queue.push(right)
      }
    }
  }
}