leetcode-----105. 从前序与中序遍历序列构造二叉树

• 链接：https://leetcode-cn.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/

代码

/**
 * 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:
    unordered_map<int, int> pos;
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        for (int i = 0; i < preorder.size(); ++i) pos[inorder[i]] = i;
        return build(preorder, inorder, 0, preorder.size() - 1, 0, inorder.size() - 1);
    }

    TreeNode* build(vector<int>& preorder, vector<int>& inorder, int pl, int pr, int il, int ir) {
        if (pl > pr) return NULL;
        auto root = new TreeNode(preorder[pl]);
        int k = pos[root->val];
        root->left = build(preorder, inorder, pl + 1, pl +k - il, il, k - 1);
        root->right = build(preorder, inorder, pl + k - il + 1, pr, k + 1, ir);
        return root;
    }
};