222.cpp

// recursion.cpp
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
  int countNodes(TreeNode *root) {
    if (!root)
      return 0;
    TreeNode *l = root, *r = root;
    int ld = 0, rd = 0;
    while (l) {
      ++ld;
      l = l->left;
    }
    while (r) {
      ++rd;
      r = r->right;
    }
    if (ld == rd)
      return (1 << ld) - 1;
    return 1 + countNodes(root->left) + countNodes(root->right);
  }
};
// somethingFaster.cpp
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution2 {
public:
  int countNodes(TreeNode *root) {
    int Deep = 0, result = 0;
    if (!root)
      return 0;
    TreeNode *temp = root, *cur = root;
    while (temp) {
      Deep++;
      temp = temp->left;
    }
    int parentHigh = Deep; //父节点的高度，也就是当前节点的高度

    while (
        parentHigh >
        1) { // parentHigh表示当前节点，因为要计算子节点即curHigh，所以不能等于0
      cur = root->left;
      int curHigh = parentHigh - 1; //当前节点的子节点高度
      for (int i = curHigh; i > 1; i--)
        cur = cur->right;
      if (cur) {
        root = root->right;
        result += 1 << (curHigh - 1);
      } else {
        root = root->left;
      }
      parentHigh--;
    }
    if (root)
      result++;
    return result + (1 << (Deep - 1)) - 1;
  }
};