0095.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:
  vector<TreeNode *> generateTrees(int n) {
    // return func1(1, n);
    return n > 0 ? func2(1, n) : vector<TreeNode *>();
  }

  vector<TreeNode *> func1(int lo, int hi) {
    vector<TreeNode *> arr;
    if (lo > hi) {
      return arr;
    }
    for (int mi = lo; mi <= hi; mi++) {
      vector<TreeNode *> lf = func1(lo, mi - 1);
      vector<TreeNode *> rt = func1(mi + 1, hi);
      if (lf.size() > 0 && rt.size() > 0) {
        for (auto left : lf) {
          for (auto right : rt) {
            TreeNode *root = new TreeNode(mi);
            root->left = left;
            root->right = right;
            arr.push_back(root);
          }
        }
      } else if (lf.size() > 0) {
        for (auto left : lf) {
          TreeNode *root = new TreeNode(mi);
          root->left = left;
          arr.push_back(root);
        }
      } else if (rt.size() > 0) {
        for (auto right : rt) {
          TreeNode *root = new TreeNode(mi);
          root->right = right;
          arr.push_back(root);
        }
      } else {
        TreeNode *root = new TreeNode(mi);
        arr.push_back(root);
      }
    }
    return arr;
  }

  // ** reference, more concise for empty return arr
  // ** but less readable
  vector<TreeNode *> func2(int lo, int hi) {
    vector<TreeNode *> arr;
    if (lo > hi) {
      arr.push_back(NULL);
      return arr;
    }

    for (int mi = lo; mi <= hi; mi++) {
      vector<TreeNode *> lf = func2(lo, mi - 1);
      vector<TreeNode *> rt = func2(mi + 1, hi);
      for (auto left : lf) {
        for (auto right : rt) {
          TreeNode *root = new TreeNode(mi);
          root->left = left;
          root->right = right;
          arr.push_back(root);
        }
      }
    }

    return arr;
  }
};