LeetCode 3550: Multi Source Flood Fill — Step-by-Step Visual Trace

Simulate multi-source BFS flood fill on a grid. Multiple colors spread simultaneously — when they collide, the maximum color wins.

Approach

Multi-source BFS. Start with all source cells. At each time step, expand to uncolored neighbors. Use a hash map to track the max color proposed for each cell. Apply winners and repeat.

Time: O(n * m) · Space: O(n * m)

Code

class Solution:
    def colorGrid(self, n: int, m: int, sources: list[list[int]]) -> list[list[int]]:
        ans = [[0] * m for _ in range(n)]

        current_layer = []
        for r, c, color in sources:
            ans[r][c] = color
            current_layer.append((r, c))

        directions = [(-1, 0), (1, 0), (0, -1), (0, 1)]

        while current_layer:
            next_layer_map = {}

            for r, c in current_layer:
                color = ans[r][c]
                for dr, dc in directions:
                    nr, nc = r + dr, c + dc
                    if 0 <= nr < n and 0 <= nc < m and ans[nr][nc] == 0:
                        if (nr, nc) not in next_layer_map or color > next_layer_map[(nr, nc)]:
                            next_layer_map[(nr, nc)] = color

            current_layer = []
            for (nr, nc), max_color in next_layer_map.items():
                ans[nr][nc] = max_color
                current_layer.append((nr, nc))

        return ans