Completed s30 Backtracking-1

combinationSum.py

@@ -0,0 +1,25 @@
+class Solution:
+    def combinationSum(self, candidates, target):
+        self.result = []
+        self.helper(candidates, target, 0, [])
+        return self.result
+
+    def helper(self, candidates, target, pivot, path):
+        #base case
+        if target < 0 or pivot == len(candidates):
+            return
+
+        if target == 0:
+            self.result.append(list(path))
+            return
+
+        for i in range(pivot, len(candidates)):
+            #action
+            path.append(candidates[i])
+            #recurse
+            self.helper(candidates, target - candidates[i], i, path)
+            #backtrack
+            path.pop()
+
+# TC - O(2 ^ (m+n)) where m is the candidates length and n is the target
+# SC - O(n)

expressionAddOperators.py

@@ -0,0 +1,28 @@
+from typing import List
+class Solution:
+    def addOperators(self, num: str, target: int) -> List[str]:
+        self.result = []
+
+        def helper(i, curr, calc, tail, path):
+            if i == len(num):
+                if calc == target and curr == 0:
+                    self.result.append(path)
+                return
+
+            curr = curr * 10 + int(num[i])
+
+            if curr > 0:
+                helper(i + 1, curr, calc, tail, path)
+
+            if not path:
+                helper(i + 1, 0, curr, curr, path + str(curr))
+            else:
+                helper(i + 1, 0, calc + curr, curr, path + "+" + str(curr))
+                helper(i + 1, 0, calc - curr, -curr, path + "-" + str(curr))
+                helper(i + 1, 0, calc - tail + (tail * curr), tail * curr, path + "*" + str(curr))
+
+        helper(0, 0, 0, 0, "")
+        return self.result
+
+# TC - O(4^n)
+# SC - O(n)