Backtracking-3 completed

nqueens.py

@@ -0,0 +1,50 @@
+def solveNQueens(self, n: int) -> List[List[str]]:
+    self.result=[]
+    board=[[False]*n for _ in range(n)]
+    self.helper(board,0,n) 
+    return self.result
+
+def helper(self,board,row,n):
+    if row==n:
+        list_=[]
+        for i in range(n):
+            sb=[]
+            for j in range(n):
+                if board[i][j]:
+                    sb.append("Q")
+                else:
+                    sb.append(".")
+            list_.append("".join(sb))
+        self.result.append(list_)
+        return
+    for j in range(n):
+        if self.issafe(board,row,j,n):
+            board[row][j]=True
+            self.helper(board,row+1,n)
+            board[row][j]=False
+
+def issafe(self,board,i,j,n):
+    r=i
+    c=j
+    while r>=0:
+        if board[r][c]:
+            return False
+        r-=1
+    r=i
+    c=j
+    while r>=0 and c>=0:
+        if board[r][c]:
+            return False
+        r-=1
+        c-=1
+    r=i
+    c=j
+    while r>=0 and c<n:
+        if board[r][c]:
+            return False
+        r-=1
+        c+=1
+    return True
+
+
+

word_search.py

@@ -0,0 +1,28 @@
+def exist(self, board: List[List[str]], word: str) -> bool:
+    self.dirs=[[-1,0],[0,-1],[0,1],[1,0]]
+    self.m=len(board)
+    self.n=len(board[0])
+
+    for i in range(self.m):
+        for j in range(self.n):
+            if self.dfs(board,i,j,word,0):
+                return True
+    return False
+
+def dfs(self,board,i,j,word,idx):
+    if idx==len(word):
+        return True
+    if i<0 or j<0 or i>=self.m or j>=self.n or board[i][j]=='#':
+        return False
+    if word[idx]!= board[i][j]:
+        return False
+    board[i][j]='#'
+
+    for dir in self.dirs:
+        r=dir[0]+i
+        c=dir[1]+j
+
+        if self.dfs(board,r,c,word,idx+1):
+            return True
+    board[i][j]=word[idx]
+    return False