add Python 21,22 solutions

Author: qinyii

Problems/21-Minimum-Suffix-Flips/Minimum_Suffix_Flips.py

@@ -0,0 +1,13 @@
+class Solution(object):
+    def minFlips(self, target):
+        """
+        :type target: str
+        :rtype: int
+        """
+        res= 0
+        s ="0"
+        for c in target:
+            if c != s:
+                res += 1
+                s = c
+        return res

Problems/22-Merge-Strings-Alternately/Merge-Strings-Alternately.py

@@ -0,0 +1,20 @@
+class Solution(object):
+    def mergeAlternately(self, word1, word2):
+        """
+        :type word1: str
+        :type word2: str
+        :rtype: str
+        """
+        l1 = len(word1)
+        l2 = len(word2)
+        i,j = 0,0
+        res = list()
+        while i < l1 or j < l2:
+            if i < l1:
+                res.append(word1[i])
+                i += 1
+            if j < l2:
+                res.append(word2[j])
+                j += 1
+        
+        return "".join(res)