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Author: wuduhren

problems/alien-dictionary.py

@@ -0,0 +1,37 @@
+"""
+Topological Sort
+"""
+class Solution(object):
+    def alienOrder(self, words):
+        #return true if cycles are detected.
+        def dfs(c):
+            if c in path: return True
+            if c in visited: return False
+            path.add(c)
+            for nei in adj[c]:
+                if dfs(nei): return True
+            res.append(c)
+            path.remove(c)
+            visited.add(c)
+            return False
+        
+        #build adjacency list
+        adj = {c: set() for word in words for c in word}
+        for i in xrange(len(words)-1):
+            w1, w2 = words[i], words[i+1]
+            minLen = min(len(w1), len(w2))
+            if w1[:minLen]==w2[:minLen] and len(w1)>len(w2): return ""
+            
+            for j in xrange(minLen):
+                if w1[j]!=w2[j]:
+                    adj[w1[j]].add(w2[j])
+                    break
+        
+        #topological sort
+        path = set() #path currently being reversed
+        visited = set() #done processing
+        res = []
+        for c in adj:
+            if dfs(c): return ""
+            
+        return "".join(reversed(res))

problems/remove-nth-node-from-end-of-list.py

@@ -0,0 +1,25 @@
+class Solution(object):
+    def removeNthFromEnd(self, head, n):
+        def getCount(node0):
+            curr = node0
+            count = 0
+            while curr:
+                curr = curr.next
+                count += 1
+            return count
+        
+        def removeNext(node0):
+            nextNode = node0.next
+            if not nextNode: return
+            node0.next = nextNode.next
+        
+        k = getCount(head)-n-1 #need to "curr = curr.next" k times to reach the node that we can call removeNext(curr)
+        if k==-1: return head.next #remove head
+        
+        curr = head
+        while k>0:
+            k -= 1
+            curr = curr.next
+        
+        removeNext(curr)
+        return head

problems/rotate-image.py

@@ -0,0 +1,16 @@
+class Solution(object):
+    def rotate(self, matrix):
+        N = len(matrix)
+        
+        #transpose
+        for i in xrange(N):
+            for j in xrange(N):
+                if j<=i: continue
+                matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
+        
+        #reflect
+        for i in xrange(N):
+            for j in xrange(N/2):
+                matrix[i][j], matrix[i][N-1-j] = matrix[i][N-1-j], matrix[i][j]
+        
+        return matrix

problems/spiral-matrix.py

@@ -0,0 +1,42 @@
+class Solution(object):
+    def spiralOrder(self, matrix):
+        ans = []
+        i, j = 0, 0
+        direction = 'right'
+        count = 0
+        
+        while count<=len(matrix)*len(matrix[0]):
+            iNext, jNext = i, j
+            
+            if direction=='right':
+                if j+1<len(matrix[0]) and matrix[i][j+1]!='v':
+                    jNext = j+1
+                else:
+                    direction = 'down'
+            elif direction=='down':
+                if i+1<len(matrix) and matrix[i+1][j]!='v':
+                    iNext = i+1
+                else:
+                    direction = 'left'
+            elif direction=='left':
+                if j-1>=0 and matrix[i][j-1]!='v':
+                    jNext = j-1
+                else:
+                    direction = 'up'
+            elif direction=='up':
+                if i-1>=0 and matrix[i-1][j]!='v':
+                    iNext = i-1
+                else:
+                    direction = 'right'
+            
+            if (iNext, jNext)!=(i, j) or count>=len(matrix)*len(matrix[0])-1:
+                ans.append(matrix[i][j])
+                matrix[i][j] = 'v' #visited
+                count += 1
+                i = iNext
+                j = jNext
+
+        return ans[:-1]
+                
+        
+