题解 | #从中序与后序遍历序列构造二叉树#

某厂笔试题

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param inorder int整型vector 中序遍历序列
     * @param postorder int整型vector 后序遍历序列
     * @return TreeNode类
     */
    int post_pos;
    unordered_map<int, int> mp;
    TreeNode* deal(int left, int right, vector<int>& inorder, vector<int>& postorder) {
        if (left > right) return NULL;
        int val = postorder[post_pos];
        int idx = mp[val];
        TreeNode *root = new TreeNode(val);
        post_pos--;
        root->right = deal(idx + 1, right, inorder, postorder);
        root->left = deal(left, idx - 1, inorder, postorder);
        return root;
    }
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        // write code here
        if (inorder.size() == 0) return NULL;
        post_pos = postorder.size() - 1;
        for (int i = 0; i < inorder.size(); i++) {
            mp[inorder[i]] = i;
        }
        return deal(0, post_pos, inorder, postorder);
    }
};