solove question 23

Author: lilyhe123

first50.py

@@ -1272,6 +1272,78 @@ def test_22():
     assert getOriginalWords(s, words) == ["bedbath", "and", "beyond"]
 
 
+"""
+question 23
+You are given an M by N matrix consisting of booleans that represents
+a board. Each True boolean represents a wall. Each False boolean
+represents a tile you can walk on.
+
+Given this matrix, a start coordinate, and an end coordinate, return
+the minimum number of steps required to reach the end coordinate from
+the start. If there is no possible path, then return null. You can move up,
+left, down, and right. You cannot move through walls.
+You cannot wrap around the edges of the board.
+
+For example, given the following board:
+
+[[f, f, f, f],
+[t, t, f, t],
+[f, f, f, f],
+[f, f, f, f]]
+and start = (3, 0) (bottom left) and end = (0, 0) (top left),
+the minimum number of steps required to reach the end is 7,
+since we would need to go through (1, 2) because there is a wall everywhere
+else on the second row.
+-----------------
+
+From starting point, try every possible paths until reaching the end point.
+Both DFS and BFS work fine here.
+But to find the minimum steps, we need to use BFS.
+"""
+
+
+def getMinSteps(matrix, starting, ending):
+    ROLS, COLS = len(matrix), len(matrix[0])
+    directions = [(-1, 0), (1, 0), (0, 1), (0, -1)]
+    queue = deque()
+    visited = [[0 for _ in range(COLS)] for _ in range(ROLS)]
+    queue.append(starting)
+    visited[starting[0]][starting[1]] = 1
+    steps = 1
+    while queue:
+        for _ in range(len(queue)):
+            r, c = queue.popleft()
+            # try all the directions to go forward
+            for dr, dc in directions:
+                newr, newc = r + dr, c + dc
+                # filter out invalid/already-visited/wall cells
+                if (
+                    newr < 0
+                    or newr >= ROLS
+                    or newc < 0
+                    or newc >= COLS
+                    or visited[newr][newc]
+                    or matrix[newr][newc] == "t"
+                ):
+                    continue
+                if (newr, newc) == ending:
+                    return steps
+                visited[newr][newc] = 1
+                queue.append((newr, newc))
+        steps += 1
+    return -1
+
+
+def test_23():
+    matrix = [
+        ["f", "f", "f", "f"],
+        ["t", "t", "f", "t"],
+        ["f", "f", "f", "f"],
+        ["f", "f", "f", "f"],
+    ]
+    assert getMinSteps(matrix, (3, 0), (0, 0)) == 7
+
+
 """
 question 32 TODO
 This problem was asked by Jane Street.