Min Stack/ Isomorphic Strings

Author: gavinfish

155 Min Stack.py

@@ -0,0 +1,63 @@
+'''
+Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
+
+push(x) -- Push element x onto stack.
+pop() -- Removes the element on top of the stack.
+top() -- Get the top element.
+getMin() -- Retrieve the minimum element in the stack.
+'''
+
+class MinStack(object):
+    def __init__(self):
+        """
+        initialize your data structure here.
+        """
+        self.stack = []
+        self.minStack = []
+
+    def push(self, x):
+        """
+        :type x: int
+        :rtype: nothing
+        """
+        self.stack.append(x)
+        if not self.minStack:
+            self.minStack.append(x)
+        else:
+            if self.minStack[-1] >= x:
+                self.minStack.append(x)
+
+    def pop(self):
+        """
+        :rtype: nothing
+        """
+        if self.stack:
+            if self.minStack[-1] == self.stack[-1]:
+                self.minStack.pop()
+            self.stack.pop()
+
+    def top(self):
+        """
+        :rtype: int
+        """
+        if self.stack:
+            return self.stack[-1]
+
+    def getMin(self):
+        """
+        :rtype: int
+        """
+        if self.minStack:
+            return self.minStack[-1]
+
+
+if __name__ == "__main__":
+    minStack = MinStack()
+    minStack.push(4)
+    minStack.push(5)
+    minStack.push(1)
+    minStack.push(3)
+    assert minStack.getMin() == 1
+    minStack.pop()
+    minStack.pop()
+    assert minStack.top() == 5

205 Isomorphic Strings.py

@@ -0,0 +1,41 @@
+'''
+Given two strings s and t, determine if they are isomorphic.
+
+Two strings are isomorphic if the characters in s can be replaced to get t.
+
+All occurrences of a character must be replaced with another character while preserving the order of characters. No two characters may map to the same character but a character may map to itself.
+
+For example,
+Given "egg", "add", return true.
+
+Given "foo", "bar", return false.
+
+Given "paper", "title", return true.
+
+Note:
+You may assume both s and t have the same length.
+'''
+
+class Solution(object):
+    def isIsomorphic(self, s, t):
+        """
+        :type s: str
+        :type t: str
+        :rtype: bool
+        """
+        found = {}
+        for c1, c2 in zip(s, t):
+            if c1 in found:
+                if not found[c1] == c2:
+                    return False
+            else:
+                if c2 in found.values():
+                    return False
+                found[c1] = c2
+        return True
+
+
+if __name__ == "__main__":
+    assert Solution().isIsomorphic("egg", "add") == True
+    assert Solution().isIsomorphic("foo", "bar") == False
+    assert Solution().isIsomorphic("paper", "title") == True