0889.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:
    TreeNode* constructFromPrePost(vector<int>& pre, vector<int>& post) {
        return helper(0, 0, pre.size(), pre, post);
    }

private:
    TreeNode* helper(int i, int j, int n, vector<int>& pre, vector<int>& post) {
        if (n == 0) return NULL;
        TreeNode* root = new TreeNode(pre[i]);
        if (n == 1) return root;

        int k = j;
        while (post[k] != pre[i + 1]) k++;
        int l = k - j + 1;

        root->left = helper(i + 1, j, l, pre, post);
        root->right = helper(i + l + 1, j + l, n - l - 1, pre, post);

        return root;
    }
};