Create shortest-path-with-alternating-colors.py

Python/shortest-path-with-alternating-colors.py

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+# Time:  O(n + e), e is the number of red and blue edges
+# Space: O(n + e)
+
+import collections
+
+
+class Solution(object):
+    def shortestAlternatingPaths(self, n, red_edges, blue_edges):
+        """
+        :type n: int
+        :type red_edges: List[List[int]]
+        :type blue_edges: List[List[int]]
+        :rtype: List[int]
+        """
+        neighbors = [[set() for _ in xrange(2)] for _ in xrange(n)]
+        for i, j in red_edges:
+            neighbors[i][0].add(j)
+        for i, j in blue_edges:
+            neighbors[i][1].add(j)
+        INF = max(2*n-3, 0)+1
+        dist = [[INF, INF] for i in xrange(n)]
+        dist[0] = [0, 0]
+        q = collections.deque([(0, 0), (0, 1)])
+        while q:
+            i, c = q.popleft()
+            for j in neighbors[i][c]:
+                if dist[j][c] != INF:
+                    continue
+                dist[j][c] = dist[i][1^c]+1
+                q.append((j, 1^c))
+        return [x if x != INF else -1 for x in map(min, dist)]