889. Construct Binary Tree from Preorder and Postorder Traversal

889. Construct Binary Tree from Preorder and Postorder Traversal (Medium)

Return any binary tree that matches the given preorder and postorder traversals.

Values in the traversals pre and post are distinct positive integers.

 

Example 1:

Input: pre = [1,2,4,5,3,6,7], post = [4,5,2,6,7,3,1]
Output: [1,2,3,4,5,6,7]

 

Note:

• 1 <= pre.length == post.length <= 30
• pre[] and post[] are both permutations of 1, 2, ..., pre.length.
• It is guaranteed an answer exists. If there exists multiple answers, you can return any of them.

Companies:
Google, Bloomberg

Related Topics:
Tree

Solution 1.

// OJ: https://leetcode.com/problems/construct-binary-tree-from-preorder-and-postorder-traversal/
// Author: github.com/lzl124631x
// Time: O(N^2)
// Space: O(N)
class Solution {
private:
    TreeNode *construct(vector<int> &pre, int preBegin, int preEnd,
                       vector<int> &post, int postBegin, int postEnd) {
        if (preBegin >= preEnd) return NULL;
        auto node = new TreeNode(pre[preBegin]);
        if (preBegin + 1 < preEnd) {
            int leftVal = pre[preBegin + 1];
            int postMid = find(post.begin() + postBegin, post.begin() + postEnd - 1, leftVal) - post.begin();
            int postLeftLength = postMid - postBegin + 1;
            node->left = construct(pre, preBegin + 1, preBegin + 1 + postLeftLength,
                                   post, postBegin, postMid + 1);
            node->right = construct(pre, preBegin + 1 + postLeftLength, preEnd,
                                   post, postMid + 1, postEnd - 1);
        }
        return node;
    }
public:
    TreeNode* constructFromPrePost(vector<int>& pre, vector<int>& post) {
        return construct(pre, 0, pre.size(), post, 0, post.size());
    }
};