LeetCode: Verify Preorder Sequence in Binary Search Tree

// https://code.dennyzhang.com/verify-preorder-sequence-in-binary-search-tree
// Basic Ideas: monotonic stack
//
//   pre-order:
//      push left sub-tree to stack
//
//   M L R
//
//  Inside stack:
//
//  R
//  L
//  M
//  
//  When get right-subtree node, pop all left-subtree nodes..
// Complexity: Time O(n), Space O(n)
func verifyPreorder(preorder []int) bool {
    stack := []int{}
    root := -1<<32
    for _, v := range preorder {
        // right sub-tree is bigger than root
        if v < root {
            return false
        }
        for len(stack) > 0 && stack[len(stack)-1] < v {
            // pop
            root = stack[len(stack)-1]
            stack = stack[0:len(stack)-1]
        }
        stack = append(stack, v)
    }
    return true
}