Description

Submission

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
    unordered_map<int, int> inorderIndex;
    TreeNode* dfs(int i, int j, int x, int y, vector<int>& inorder, vector<int>& postorder) {
        if(j < i) return nullptr;
        if(j == i) return new TreeNode(inorder[i]);
        int rootVal = postorder[y];
        int inorderRootIndex = inorderIndex[rootVal];
        // for inorder
        int lenLeft = inorderRootIndex - i;
        int lenRight = j - inorderRootIndex;

        return new TreeNode(
            rootVal,
            dfs(i, inorderRootIndex - 1, x, x + lenLeft - 1, inorder, postorder),
            dfs(inorderRootIndex + 1, inorderRootIndex + lenRight, y - lenRight, y - 1, inorder, postorder)
        );
    }
public:
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        int n = inorder.size();
        for(int i = 0; i < n; ++i) {
            inorderIndex[inorder[i]] = i;
        }

        return dfs(0, n-1, 0, n-1, inorder, postorder);
    }
};


// [9],3,[15,20,7]