Graph Exploration Target: 15s
Initialize queue with all sources. BFS radiates outward in parallel from all starting points.
q = deque()
for i in range(R):
for j in range(C):
if grid[i][j] == 2: q.append((i, j, 0))
while q:
i, j, t = q.popleft()
for di, dj in ((1,0),(-1,0),(0,1),(0,-1)):
ni, nj = i+di, j+dj
if 0<=ni<R and 0<=nj<C and grid[ni][nj]==1:
grid[ni][nj] = 2
q.append((ni, nj, t+1))Type it from memory. Go.