two pointers

Interleaving Positive and Negative Numbers.py

@@ -0,0 +1,48 @@
+"""
+Given an array with positive and negative integers. Re-range it to interleaving with positive and negative integers.
+
+Have you met this question in a real interview? Yes
+Example
+Given [-1, -2, -3, 4, 5, 6], after re-range, it will be [-1, 5, -2, 4, -3, 6] or any other reasonable answer.
+
+Note
+You are not necessary to keep the original order of positive integers or negative integers.
+
+Challenge
+Do it in-place and without extra memory.
+"""
+__author__ = 'Daniel'
+
+
+class Solution(object):
+    def rerange(self, A):
+        """
+        Algorithm: Two Pointers
+        :type A: List[int]
+        :rtype: None, in-place
+        """
+        n = len(A)
+        pos_cnt = len(filter(lambda x: x > 0, A))
+        # expecting positive
+        pos_expt = True if pos_cnt*2 > n else False
+
+        neg = 0  # next negative
+        pos = 0  # next positive
+        for i in xrange(n):
+            while neg < n and A[neg] > 0: neg += 1
+            while pos < n and A[pos] < 0: pos += 1
+            if pos_expt:
+                A[i], A[pos] = A[pos], A[i]
+            else:
+                A[i], A[neg] = A[neg], A[i]
+
+            if i == neg: neg += 1
+            if i == pos: pos += 1
+
+            pos_expt = not pos_expt
+
+
+if __name__ == "__main__":
+    A = [-33, -19, 30, 26, 21, -9]
+    Solution().rerange(A)
+    assert A == [-33, 30, -19, 26, -9, 21]