added nested brackets problem

Author: alvin562

other/nested_brackets.py

@@ -0,0 +1,57 @@
+'''
+The nested brackets problem is a problem that determines if a sequence of
+brackets are properly nested.  A sequence of brackets s is considered properly nested
+if any of the following conditions are true:
+
+	- s is empty
+	- s has the form (U) or [U] or {U} where U is a properly nested string
+	- s has the form VW where V and W are properly nested strings
+
+For example, the string "()()[()]" is properly nested but "[(()]" is not.
+
+The function called is_balanced takes as input a string S which is a sequence of brackets and
+returns true if S is nested and false otherwise.
+
+'''
+
+
+def is_balanced(S):
+
+    stack = []
+    
+    for i in range(len(S)):
+        
+        if S[i] == '(' or S[i] == '{' or S[i] == '[':
+            stack.append(S[i])
+            
+        else:
+            
+            if len(stack) > 0:
+                
+                pair = stack.pop() + S[i]
+                
+                if pair != '[]' and pair != '()' and pair != '{}':
+                    return False
+                
+            else:
+                return False
+                
+    if len(stack) == 0:
+        return True
+        
+    return False
+
+
+def main():
+
+    S = input("Enter sequence of brackets within quotes: ")
+
+    if is_balanced(S):
+        print S, "is balanced"
+    
+    else:
+	print S, "is not balanced"
+
+
+if __name__ == "__main__":
+    main()