add solution of 120

120. Triangle
- https://leetcode.com/problems/triangle/
- https://leetcode.com/explore/featured/card/april-leetcoding-challenge-2021/595/week-3-april-15th-april-21st/3715/

Author: rushirg

solutions/triangle.py

@@ -5,7 +5,7 @@
 """
 
 # Method 1
-# Time: O(N)
+# Time: O(N), modify triangle in-place
 
 class Solution(object):
     def minimumTotal(self, triangle):
@@ -23,3 +23,28 @@ def minimumTotal(self, triangle):
                     triangle[i][j] = min(triangle[i - 1][j - 1] + triangle[i][j], triangle[i - 1][j] + triangle[i][j])
             print(triangle[i])
         return min(triangle[-1])
+
+
+
+# Method 2
+# Space: O(N) without modifying original triangle
+class Solution(object):
+    def minimumTotal(self, triangle):
+        """
+        :type triangle: List[List[int]]
+        :rtype: int
+        """
+        prevRow = triangle[0]
+        for i in range(1, len(triangle)):
+            currRow = []
+            for j in range(len(triangle[i])):
+                smallAbove = float('-inf')
+                if j == 0:
+                    smallAbove =  prevRow[j]
+                elif j == len(triangle[i]) - 1:
+                    smallAbove = prevRow[j - 1]
+                else:
+                    smallAbove = min(prevRow[j-1], prevRow[j])
+                currRow.append(smallAbove + triangle[i][j])
+            prevRow = currRow
+        return min(prevRow)