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Problem solution · Python

ABC405 D — Escape Route

ABC405 D — Escape Route: a Python solution using breadth-first search. Learn the idea, check the complexity, and read the full code, with credit to KATO-Hiro AtCoder Solutions.

Technique
Breadth-first search
Source
KATO-Hiro AtCoder Solutions
Length
69 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

Breadth-first search

For ABC405 D — Escape Route, the implementation explores reachable states in layers, which is the standard shape for unweighted shortest paths and minimum-step transitions.

  1. Model each valid configuration as a state and each legal move as an edge.
  2. Seed the queue with the starting state and mark it immediately.
  3. Expand each state once, recording distance or reachability for unseen neighbours.

Code notes

  • 69 lines of Python from the credited upstream file abc405_d.py.
  • The implementation visibly relies on sequence storage, ordered lookup, work queue.
  • No explicit loop blocks detected.

Complexity

Verify that each state and transition is processed only a bounded number of times; that determines the traversal cost.

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 codeABC405 D — Escape Route · PythonPython
Use this to learn the idea, then write your own version.
# -*- coding: utf-8 -*-  def main():    import sys    from collections import deque     input = sys.stdin.readline     h, w = map(int, input().split())    # TODO: Change input format if needs.    grid = [list(input().rstrip()) for _ in range(h)]    d = deque()    pending = -1    dist = [[pending] * w for _ in range(h)]     for i in range(h):        for j in range(w):            if grid[i][j] == "E":                d.append((i, j))                dist[i][j] = 0  # Initialize     visited = [[False] * w for _ in range(h)]    dxy = [(-1, 0), (1, 0), (0, -1), (0, 1)]     while d:        y, x = d.popleft()         if dist[y][x] == pending:            continue         if visited[y][x]:            continue         visited[y][x] = True         for dx, dy in dxy:            nx = x + dx            ny = y + dy             if nx < 0 or nx >= w or ny < 0 or ny >= h:                continue            if visited[ny][nx]:                continue            if grid[ny][nx] == "#":                continue            if dist[ny][nx] != pending and dist[ny][nx] <= dist[y][x]:                continue             if dy == 1:                grid[ny][nx] = "^"            elif dy == -1:                grid[ny][nx] = "v"            elif dx == 1:                grid[ny][nx] = "<"            elif dx == -1:                grid[ny][nx] = ">"             dist[ny][nx] = dist[y][x] + 1  # Update ans            d.append((ny, nx))     for i in range(h):        ans = "".join(grid[i])        print(ans)  if __name__ == "__main__":    main()

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