Clone Graph/ Gas Station

Author: gavinfish

133 Clone Graph.py

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+'''
+Clone an undirected graph. Each node in the graph contains a label and a list of its neighbors.
+
+
+OJ's undirected graph serialization:
+Nodes are labeled uniquely.
+
+We use # as a separator for each node, and , as a separator for node label and each neighbor of the node.
+As an example, consider the serialized graph {0,1,2#1,2#2,2}.
+
+The graph has a total of three nodes, and therefore contains three parts as separated by #.
+
+First node is labeled as 0. Connect node 0 to both nodes 1 and 2.
+Second node is labeled as 1. Connect node 1 to node 2.
+Third node is labeled as 2. Connect node 2 to node 2 (itself), thus forming a self-cycle.
+Visually, the graph looks like the following:
+
+       1
+      / \
+     /   \
+    0 --- 2
+         / \
+         \_/
+'''
+
+# Definition for a undirected graph node
+class UndirectedGraphNode(object):
+    def __init__(self, x):
+        self.label = x
+        self.neighbors = []
+
+
+class Solution(object):
+    def cloneGraph(self, node):
+        """
+        :type node: UndirectedGraphNode
+        :rtype: UndirectedGraphNode
+        """
+        if not node:
+            return node
+        visited = {}
+        first = UndirectedGraphNode(node.label)
+        visited[node.label] = first
+        stack = [node]
+        while stack:
+            top = stack.pop()
+            for n in top.neighbors:
+                if n.label not in visited:
+                    visited[n.label] = UndirectedGraphNode(n.label)
+                    stack.append(n)
+                visited[top.label].neighbors.append(visited[n.label])
+        return first
+
+
+if __name__ == "__main__":
+    None

134 Gas Station.py

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+'''
+There are N gas stations along a circular route, where the amount of gas at station i is gas[i].
+
+You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from station i to its next station (i+1). You begin the journey with an empty tank at one of the gas stations.
+
+Return the starting gas station's index if you can travel around the circuit once, otherwise return -1.
+
+Note:
+The solution is guaranteed to be unique.
+'''
+
+class Solution(object):
+    def canCompleteCircuit(self, gas, cost):
+        """
+        :type gas: List[int]
+        :type cost: List[int]
+        :rtype: int
+        """
+        if sum(gas) < sum(cost):
+            return -1
+        min_sum, min_index, total = 0, 0, 0
+        for i in range(len(gas)):
+            total += gas[i] - cost[i]
+            if min_sum > total:
+                min_sum, min_index = total, i + 1
+        return -1 if total < 0 else min_index
+
+
+if __name__ == "__main__":
+    assert Solution().canCompleteCircuit([5], [4]) == 0
+    assert Solution().canCompleteCircuit([5, 1, 2, 3, 4], [4, 4, 1, 5, 1]) == 4