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Problem solution · Python

ABC217 D — Cutting Woods

ABC217 D — Cutting Woods: a Python solution using binary search. Learn the idea, check the complexity, and read the full code, with credit to KATO-Hiro AtCoder Solutions.

Technique
Binary search
Source
KATO-Hiro AtCoder Solutions
Length
226 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Binary search

For ABC217 D — Cutting Woods, the implementation exploits a monotonic condition to discard half of the remaining search space after every check.

  1. Identify the ordered answer range or sorted search domain.
  2. Write a predicate whose truth changes only once.
  3. Move the appropriate boundary after each midpoint check and return the final feasible position.

Code notes

  • 226 lines of Python from the credited upstream file abc217_d.py.
  • The implementation visibly relies on sequence storage, ordered lookup.
  • No explicit loop blocks detected.

Complexity

Multiply the logarithmic number of midpoint checks by the cost of one predicate evaluation.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from KATO-Hiro AtCoder Solutions by KATO-Hiro and is used under the CC0-1.0 licence.

Open problem ↗Original source ↗
Full codeABC217 D — Cutting Woods · PythonPython
Use this to learn the idea, then write your own version.
# -*- coding: utf-8 -*- class BalancingTree:    """    Self-balancing binary search tree using pivot values.     See:    https://qiita.com/Kiri8128/items/6256f8559f0026485d90    """     def __init__(self, n):        self.N = n        self.root = self.node(1 << n, 1 << n)     def append(self, v): # Note: v does not exist in the tree.        v += 1        nd = self.root         while True:            if v == nd.value:                # v is already in the tree.                # TODO: Write code here, if do something.                return 0            else:                mi, ma = min(v, nd.value), max(v, nd.value)                 if mi < nd.pivot:                    nd.value = ma                     if nd.left:                        nd = nd.left                        v = mi                    else:                        p = nd.pivot                        nd.left = self.node(mi, p - (p & -p) // 2)                        break                else:                    nd.value = mi                     if nd.right:                        nd = nd.right                        v = ma                    else:                        p = nd.pivot                        nd.right = self.node(ma, p + (p & -p) // 2)                        break     def leftmost(self, nd):        if nd.left:             return self.leftmost(nd.left)        return nd     def rightmost(self, nd):        if nd.right:             return self.rightmost(nd.right)        return nd     def find_l(self, v):        """The maximum value among the values truly less than v (If not, -1).        """         v += 1        nd = self.root        prev = 0         if nd.value < v:             prev = nd.value         while True:            if v <= nd.value:                if nd.left:                    nd = nd.left                else:                    return prev - 1            else:                prev = nd.value                 if nd.right:                    nd = nd.right                else:                    return prev - 1     def find_r(self, v):        """The smallest value among the values truly greater than v         (if not, root).        """         v += 1        nd = self.root        prev = 0         if nd.value > v:             prev = nd.value         while True:            if v < nd.value:                prev = nd.value                 if nd.left:                    nd = nd.left                else:                    return prev - 1            else:                if nd.right:                    nd = nd.right                else:                    return prev - 1     @property    def max(self):        return self.find_l((1 << self.N) - 1)     @property    def min(self):        return self.find_r(-1)     def delete(self, v, nd = None, prev = None):        v += 1         if not nd:             nd = self.root        if not prev:             prev = nd         while v != nd.value:            prev = nd             if v <= nd.value:                if nd.left:                    nd = nd.left                else:                    #####                    return            else:                if nd.right:                    nd = nd.right                else:                    #####                    return         if (not nd.left) and (not nd.right):            if not prev.left:                prev.right = None            elif not prev.right:                prev.left = None            else:                if nd.pivot == prev.left.pivot:                    prev.left = None                else:                    prev.right = None         elif nd.right:            # print("type A", v)            nd.value = self.leftmost(nd.right).value            self.delete(nd.value - 1, nd.right, nd)            else:            # print("type B", v)            nd.value = self.rightmost(nd.left).value            self.delete(nd.value - 1, nd.left, nd)     def __contains__(self, v: int) -> bool:        return self.find_r(v - 1) == v     class node:        def __init__(self, v, p):            self.value = v            self.pivot = p            self.left = None            self.right = None     def debug(self):        def debug_info(nd_):            return (nd_.value - 1, nd_.pivot - 1, nd_.left.value - 1 if nd_.left else -1, nd_.right.value - 1 if nd_.right else -1)         def debug_node(nd):            re = []             if nd.left:                re += debug_node(nd.left)            if nd.value:                 re.append(debug_info(nd))            if nd.right:                re += debug_node(nd.right)            return re         print("Debug - root =", self.root.value - 1, debug_node(self.root)[:50])     def debug_list(self):        def debug_node(nd):            re = []             if nd.left:                re += debug_node(nd.left)            if nd.value:                 re.append(nd.value - 1)            if nd.right:                re += debug_node(nd.right)            return re        return debug_node(self.root)[:-1]  def main():    import sys     input = sys.stdin.readline     l, q = map(int, input().split())    bt = BalancingTree(31) # 0 ～ 2 ** 31 - 1 までの要素を入れられるピボット木    bt.append(0)    bt.append(l)     for _ in range(q):        ci, qi = map(int, input().split())         if ci == 1:            bt.append(qi)        else:            left = bt.find_l(qi)            right = bt.find_r(qi)             print(right - left)  if __name__ == "__main__":    main()

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