2020-03-08

Author: JiayangWu

1377.T秒后青蛙的位置/1377-T秒后青蛙的位置.py

@@ -0,0 +1,42 @@
+class Solution(object):
+    def frogPosition(self, n, edges, t, target):
+        """
+        :type n: int
+        :type edges: List[List[int]]
+        :type t: int
+        :type target: int
+        :rtype: float
+        """
+        from collections import deque, defaultdict
+        
+        # 1. 建图
+        adj = defaultdict(set)
+        for s, e in edges:
+            adj[s].add(e)
+            adj[e].add(s)
+        
+        # 2. 初始化 DP 数组
+        # dp[k][node] 代表在第 k 秒，蛤处于 node 的概率
+        # dp[k][node] += dp[k - 1][parent] * prbability(parent -> node)
+        
+        dp = [[0 for _ in range(n + 1)] for _ in range(t + 1)]
+        dp[0][1] = 1
+        
+        for time in range(1, t + 1):
+            for par in range(1, n + 1):
+                if dp[time - 1][par]:
+                    if not adj[par]:
+                        # 如果无处可去，则停留在原地
+                        dp[time][par] = dp[time - 1][par]
+                    else:
+                        # 能跳就跳
+                        for node in adj[par]:
+                            dp[time][node] += dp[time - 1][par] * 1.0 / len(adj[par])
+                            
+                        # 跳完就把用过的边删掉
+                        for node in adj[par]:
+                            adj[node].remove(par)
+                        adj[par] = set()
+
+        return dp[t][target]
+