24.bstFromPreorder.cpp

/**
 * 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 {
public:
//     TreeNode* helper(const vector<int>& preorder, int &id, int limit){
//         if(id == preorder.size() || preorder[id] > limit) return NULL;
        
//         TreeNode *root = new TreeNode(preorder[id]);
//         id++;
//         root->left = helper(preorder, id, preorder[id]);
//         root->right = helper(preorder, id, limit);
//         return root;    
//     }
    
    int i = 0;
    
    TreeNode* bstFromPreorder(vector<int>& A, int bound = INT_MAX) {
        if (i == A.size() || A[i] > bound) return NULL;
        TreeNode* root = new TreeNode(A[i++]);
        root->left = bstFromPreorder(A, root->val);
        root->right = bstFromPreorder(A, bound);
        return root;
    }
    
};