Leetcode 106. 从中序与后序遍历序列构造二叉树

题目

Leetcode 106. 从中序与后序遍历序列构造二叉树

代码（9.18 首刷自解）

class Solution {
public:
    unordered_map<int, int> val2Index;
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        for(int i = 0; i < inorder.size(); i++)
            val2Index[inorder[i]] = i;
        return build(inorder, 0, inorder.size()-1, postorder, 0, postorder.size()-1);
    }
    TreeNode* build(vector<int>& inorder, int inL, int inR, vector<int>& postorder, int pL, int pR) {
        if(inL > inR || pL > pR)
            return nullptr;
        auto root = new TreeNode(postorder[pR]);
        int index = val2Index[root->val];
        int len = index-inL;
        root->left = build(inorder, inL, index-1, postorder, pL, pL+len-1);
        root->right = build(inorder, index+1, inR, postorder, pL+len, pR-1);
        return root;
    }
};