2764. is Array a Preorder of Some ‌Binary Tree

Given a 0-indexed integer 2D array nodes, your task is to determine if the given array represents the preorder traversal of some binary tree.

For each index i, nodes[i] = [id, parentId], where id is the id of the node at the index i and parentId is the id of its parent in the tree (if the node has no parent, then parentId = -1).

Return true if the given array represents the preorder traversal of some tree, and false otherwise.

Note: the preorder traversal of a tree is a recursive way to traverse a tree in which we first visit the current node, then we do the preorder traversal for the left child, and finally, we do it for the right child.

class Solution {
public:
    bool isPreorder(vector<vector<int>>& nodes) {
        if(nodes[0][1] != -1) return false;
        vector<int> st{nodes[0][0]};
        for(int i = 1; i < nodes.size(); i++) {
            while(st.size() and st.back() != nodes[i][1]) st.pop_back();
            if(st.size() == 0) return false;
            st.push_back(nodes[i][0]);
        }
        
        return true;
    }
};