222.完全二叉树的节点个数 ​

/**
 * @param {TreeNode} root
 * @return {number}
 */
var countNodes = function (root) {
  if (!root) return 0;
  let left = root.left;
  let right = root.right;
  let leftHeight = 0;
  let rightHeight = 0;
  while (left) {
    left = left.left;
    leftHeight++;
  }
  while (right) {
    right = right.right;
    rightHeight++;
  }
  // 如果是个满二叉树，最左边和最右边的节点高度相同
  if (leftHeight === rightHeight) {
    // 满二叉树节点个数  2^k - 1
    return Math.pow(2, rightHeight + 1) - 1;
  }
  return countNodes(root.left) + countNodes(root.right) + 1;
};

/**
 * @param {TreeNode} root
 * @return {number}
 */
var countNodes = function (root) {
  if (!root) return 0;
  let left = root.left;
  let right = root.right;
  let leftHeight = 0;
  let rightHeight = 0;
  while (left) {
    left = left.left;
    leftHeight++;
  }
  while (right) {
    right = right.right;
    rightHeight++;
  }
  // 如果是个满二叉树，最左边和最右边的节点高度相同
  if (leftHeight === rightHeight) {
    // 满二叉树节点个数  2^k - 1
    return Math.pow(2, rightHeight + 1) - 1;
  }
  return countNodes(root.left) + countNodes(root.right) + 1;
};