[WXM] LeetCode 894. All Possible Full Binary Trees C++_leetcode894

#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]]
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2

Explanation:

Note:

• 1 <= N <= 20

Approach

1. 题目大意记录所有可能的满二叉树的情况。我们枚举它左右子树会有多少个结点即可，然后不断递归枚举它左右子树会有多少个结点下去，最后再一一返回。

Code

class Solution {
public:
	vector<TreeNode*> allPossibleFBT(int N) {
		if (N % 2==0)return{};
		if (N == 1)return{ new TreeNode(0) };
		vector<TreeNode*> ans;
		for (int i = 1; i < N; i += 2) {
			for (TreeNode* &L : allPossibleFBT(i)) {
				for (TreeNode* &R : allPossibleFBT(N - i - 1)) {
					TreeNode *root = new TreeNode(0);
					root->left = L;
					root->right = R;
					ans.push_back(root);
				}
			}
		}
		return ans;
	}
};
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