📅  最后修改于: 2023-12-03 15:25:55.608000             🧑  作者: Mango
回溯算法是一种解决问题的算法,通常用于在给定条件下尝试所有可能的结果。在这里,我们将介绍排名前20位的回溯算法面试问题,这将有助于您为面试做好准备。
八皇后问题是回溯算法中最常见的问题之一。这个问题是要在一个8x8的国际象棋棋盘上,摆放8个皇后,要求每个皇后都不能攻击到其他的皇后。
代码示例:
def is_valid(board, row, col):
for i in range(row):
if board[i] == col or \
abs(board[i] - col) == abs(i - row):
return False
return True
def n_queens_helper(n, row, board, results):
if row == n:
results.append(board[:])
return
for col in range(n):
if is_valid(board, row, col):
board[row] = col
n_queens_helper(n, row+1, board, results)
def n_queens(n):
results = []
board = [-1] * n
n_queens_helper(n, 0, board, results)
return results
给定一个仅包含数字2-9的字符串,返回所有它能表示的字母组合。
代码示例:
def letter_combinations_helper(digits, idx, path, results, mapping):
if idx == len(digits):
results.append(''.join(path))
return
for ch in mapping[digits[idx]]:
path.append(ch)
letter_combinations_helper(digits, idx+1, path, results, mapping)
path.pop()
def letter_combinations(digits):
if len(digits) == 0:
return []
mapping = {
'2': 'abc',
'3': 'def',
'4': 'ghi',
'5': 'jkl',
'6': 'mno',
'7': 'pqrs',
'8': 'tuv',
'9': 'wxyz'
}
results = []
letter_combinations_helper(digits, 0, [], results, mapping)
return results
给定一个无重复元素的数组candidates和一个目标数target,找出candidates中所有可以使数字和为target的组合。
代码示例:
def combination_sum_helper(candidates, start, target, path, results):
if target == 0:
results.append(path[:])
return
for i in range(start, len(candidates)):
if candidates[i] > target:
continue
path.append(candidates[i])
combination_sum_helper(candidates, i, target-candidates[i], path, results)
path.pop()
def combination_sum(candidates, target):
results = []
candidates.sort()
combination_sum_helper(candidates, 0, target, [], results)
return results
其他排名前20的回溯算法问题包括:
通过深入学习这些问题和对应的算法,您将在面试时更加从容,为找到理想的工作打下坚实基础。