Create find-a-good-subset-of-the-matrix.py

Author: kamyu104

Python/find-a-good-subset-of-the-matrix.py

@@ -0,0 +1,46 @@
+# Time:  O(m * 2^n)
+# Space: O(2^n)
+
+# bitmasks, constructive algorithms, greedy, submask enumeration
+class Solution(object):
+    def goodSubsetofBinaryMatrix(self, grid):
+        """
+        :type grid: List[List[int]]
+        :rtype: List[int]
+        """
+        lookup = {}
+        for i in xrange(len(grid)):
+            mask = reduce(lambda mask, j: mask|(grid[i][j]<<j), xrange(len(grid[0])), 0)
+            if not mask:
+                return [i]
+            mask2 = ((1<<len(grid[0]))-1)^mask
+            submask = mask2  # submask enumeration
+            while submask > 0:
+                if submask in lookup:
+                    return [lookup[submask], i]
+                submask = (submask-1)&mask2                
+            lookup[mask] = i
+        return []
+
+
+# Time:  O(m * 2^n)
+# Space: O(2^n)
+# bitmasks, constructive algorithms, greedy
+class Solution2(object):
+    def goodSubsetofBinaryMatrix(self, grid):
+        """
+        :type grid: List[List[int]]
+        :rtype: List[int]
+        """
+        lookup = {}
+        for i in xrange(len(grid)):
+            mask = reduce(lambda mask, j: mask|(grid[i][j]<<j), xrange(len(grid[0])), 0)
+            if not mask:
+                return [i]
+            for mask2 in xrange(1, 1<<len(grid[0])):
+                if mask&mask2:
+                    continue
+                if mask2 in lookup:
+                    return [lookup[mask2], i]
+            lookup[mask] = i
+        return []