added 0118_pascals_triangle.py

Author: nyierr

easy/0118_pascals_triangle.py

@@ -0,0 +1,49 @@
+"""
+Given an integer numRows, return the first numRows of Pascal's triangle.
+
+In Pascal's triangle, each number is the sum of the two numbers directly
+above it as shown:
+
+Example 1:
+  Input: numRows = 5
+  Output: [[1],[1,1],[1,2,1],[1,3,3,1],[1,4,6,4,1]]
+    
+Example 2:
+  Input: numRows = 1
+  Output: [[1]]
+
+Constraints:
+  * 1 <= numRows <= 30
+"""
+
+class Solution:
+    # O(n^2) solution, iterative
+    #def generate(self, numRows: int) -> List[List[int]]:        
+    #    out = [[1]]
+    #    for i in range(numRows - 1):
+    #        prev = out[-1]
+    #        curr = [prev[0]]
+    #        
+    #        for j in range(1, len(prev)):
+    #            curr.insert(j, prev[j] + prev[j-1])
+    #        
+    #        curr.append(prev[-1])
+    #        out.append(curr)
+    #        
+    #    return out
+
+    # O(n^2) solution, recursive
+    def generate(self, numRows: int) -> List[List[int]]:        
+        if numRows == 1:
+            return [[1]]
+        
+        res = self.generate(numRows - 1)
+        
+        prev = res[-1]
+        curr = [prev[0]]
+        for i in range(1, len(prev)):
+            curr.insert(i, prev[i] + prev[i-1])
+        curr.append(prev[-1])
+        
+        return res + [curr]
+