LC 894. All Possible Full Binary Trees

A full binary tree is a binary tree where each node has exactly 0 or 2 children.

Return a list of all possible full binary trees with N nodes.  Each element of the answer is the root node of one possible tree.

Each node of each tree in the answer must have node.val = 0.

You may return the final list of trees in any order.

 

Example 1:

Input: 7
Output: [[0,0,0,null,null,0,0,null,null,0,0],[0,0,0,null,null,0,0,0,0],[0,0,0,0,0,0,0],[0,0,0,0,0,null,null,null,null,0,0],[0,0,0,0,0,null,null,0,0]]
Explanation:

Runtime: 104 ms, faster than 53.23% of C++ online submissions for All Possible Full Binary Trees.

用map来优化。

class Solution {
private:
  unordered_map<int,vector<TreeNode*>> mp;
public:
  vector<TreeNode*> allPossibleFBT(int N) {
    vector<TreeNode*> ret, leftvec, rightvec;
    if(N & 1 == 0 || N <= 0) return ret;
    return helper(N);
  }
  vector<TreeNode*> helper(int N){
    vector<TreeNode*> leftvec, rightvec, ret;
    if(mp.count(N)) return mp[N];
    if(N == 1) return {new TreeNode(0)};
    for(int i=1; i < N; i+=2){
      leftvec = helper(i);
      rightvec = helper(N - i -1);
      //cout << leftvec.size() << rightvec.size() << endl;
      for(int l = 0; l<leftvec.size();l++){
        for(int r = 0; r<rightvec.size();r++){
          TreeNode* tmp = new TreeNode(0);
          tmp->left = leftvec[l];
          tmp->right = rightvec[r];
          ret.push_back(tmp);
        }
      }
    }
    //cout << ret.size() << endl;
    mp[N] = ret;
    return ret;
  }
};