finished

Problem 2.py

@@ -0,0 +1,35 @@
+ROWS, COLS = len(board), len(board[0])
+    sofar = set()
+
+    def dfs(i, j, index):
+        if index >= len(word):
+            return True
+
+        if not 0 <= i < ROWS or not 0 <= j < COLS:
+            return False
+
+        if board[i][j] != word[index]:
+            return False
+
+        if (i, j) in sofar:
+            return False
+
+        sofar.add((i, j))
+        res = (
+            dfs(i + 1, j, index + 1) or
+            dfs(i - 1, j, index + 1) or
+            dfs(i, j + 1, index + 1) or
+            dfs(i, j - 1, index + 1)
+        )
+
+        if not res:
+            sofar.remove((i, j))
+
+        return res
+
+    for i in range(ROWS):
+        for j in range(COLS):
+            if dfs(i, j, 0):  # dfs with backtracking
+                return True
+
+    return False

Problem1.py

@@ -0,0 +1,51 @@
+class Solution:
+    def solveNQueens(self, n: int) -> List[List[str]]:
+
+        res = []
+        def okay_to_place(r,c, sofar):
+            if not sofar:
+                return True
+            restricted_rows = set([i for i, j in sofar])
+            restricted_cols = set([j for i, j in sofar])
+
+            if r in restricted_rows or c in restricted_cols: #checked for same row or same col
+                return False 
+
+            for i, j in sofar:
+                if abs(i-r) == abs(j-c):
+                    return False
+            return True #or False
+
+        def form_a_board(sofar):
+            board = ["."*n for _ in range(n)]
+            for i, j in sofar:
+                board[i][j] = "Q"
+            return board
+
+        def dfs(r, sofar, q):
+            if q == 0:
+                #res.append(sofar.copy())
+                #print(sofar)
+                board = [["."]*n for _ in range(n)]
+                for i, j in sofar:
+                    board[i][j] = "Q"
+
+                board = ["".join(i) for i in board]
+                res.append(board)
+                return
+            if not 0 <= r < n:
+                return
+
+
+            for c in range(n):
+                if okay_to_place(r, c, sofar) and q > 0:
+                    sofar.append((r,c))
+                    dfs(r+1, sofar, q-1)
+                    sofar.pop()
+
+            return
+
+        dfs(0, [], n)
+        return res
+
+