Verify Preorder Sequence in Binary Search Tree_1.cpp

class Solution {
public:
    bool verifyPreorder(vector<int>& preorder) {
        int len = preorder.size(), i = 0, leftBound = INT_MIN;
        stack<int> st;
        while(i < len)
        {
            int num = preorder[i];
            if(st.size() && num < leftBound)
                return false;
            if(st.empty() || st.top() > num)
                st.push(num);
            else
            {
                while(st.size() && st.top() < num)
                {   
                    leftBound = st.top();
                    st.pop();
                }
                st.push(num);
            }
            ++i;
        }
        return true;
    }
};