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Author: Chris Wu

problems/climbing-stairs.py

@@ -43,4 +43,24 @@ def factorial(int_num):
                 int_num-=1
             return counter
 
-        return factorial(m)/(factorial(n)*factorial(m-n))
+        return factorial(m)/(factorial(n)*factorial(m-n))
+
+#2020/11/9
+class Solution(object):
+    def climbStairs(self, n):
+        def helper(n):
+            if n in history: return history[n]
+            
+            if n==0 or n==1:
+                history[n] = 1
+            elif n==2:
+                return 2
+            elif n>2:
+				#combination count of n stairs equals to
+				#(the combination after you make 1 step as first move) + (the combination after you make 2 steps as first move)
+                history[n] = helper(n-1) + helper(n-2)
+            
+            return history[n]
+            
+        history = {}
+        return helper(n)

problems/min-cost-climbing-stairs.py

@@ -20,4 +20,23 @@ def minCostClimbingStairs(self, cost):
             else:
                 memo[i] = min(memo[i-1]+cost[i-1], memo[i-2]+cost[i-2])
 
-        return memo[-1]
+        return memo[-1]
+
+#2020/11/9
+class Solution(object):
+    def minCostClimbingStairs(self, cost):
+        def helper(i):
+            if i in history: return history[i]
+            
+            if i>len(cost)-1:
+                history[i] = 0
+            elif i==len(cost)-1 or i==len(cost)-2:
+                history[i] = cost[i]
+            elif i<len(cost)-2:
+                history[i] = cost[i] + min(helper(i+1), helper(i+2))
+                
+            return history[i]
+        
+        history = {}
+        return min(helper(0), helper(1))
+            

problems/reconstruct-itinerary.py

@@ -0,0 +1,48 @@
+#build graph with departure city as key and (destination, ticket_id) as value
+#G = build_graph()
+#itinerary = ['JFK']
+#ticket_used = set()
+#dfs()
+    #if len(itinerary)==len(tickets)+1: return True
+    #here = itinerary[-1]
+    #if here not in G: return False
+    #candidates = nei, ticket_id for nei, ticket_id in G[here] if ticket_id not in ticket_used
+    #for nei, ticket_id in candidates:
+        #ticket_used.add(ticket_id)
+        #itinerary.append(nei)
+        #if dfs(): return True
+        #ticket_used.remove(ticket_id)
+        #itinerary.pop()
+    #return False
+
+import collections
+
+class Solution(object):
+    def findItinerary(self, tickets):
+        def dfs():
+            if len(itinerary)==len(tickets)+1: return True
+            here = itinerary[-1]
+            if here not in G: return False
+
+            candidates = [(nei, ticket_id) for nei, ticket_id in G[here] if ticket_id not in ticket_used]
+            
+            for nei, ticket_id in candidates:
+                ticket_used.add(ticket_id)
+                itinerary.append(nei)
+                if dfs(): return True
+                ticket_used.remove(ticket_id)
+                itinerary.pop()
+            return False
+
+        G = collections.defaultdict(list)
+        itinerary = ['JFK']
+        ticket_used = set()
+
+        #build graph
+        ticket_id_counter = 0
+        for n1, n2 in tickets:
+            G[n1].append((n2, ticket_id_counter))
+            ticket_id_counter += 1
+        for k in G: G[k].sort(key=lambda x:x[0])
+        
+        return dfs()