Shortest Bridge

In a given 2D binary array A, there are two islands. (An island is a 4-directionally connected group of 1s not connected to any other 1s.)

Now, we may change 0s to 1s so as to connect the two islands together to form 1 island.

Return the smallest…

N-Queens

The n-queens puzzle is the problem of placing n queens on an n x n chessboard such that no two queens attack each other.

Given an integer n, return all distinct solutions to the n-queens puzzle.

Each solution contains a distinct board configuration of the n-queens’ placement, where 'Q'

Given two integers n and k, return all possible combinations of k numbers out of 1 … n.

You may return the answer in any order.

Example 1:

Input: n = 4, k = 2
Output:
[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]

Example 2:

Input: n = 1…

Given an array nums of distinct integers, return all the possible permutations. You can return the answer in any order.

Example 1:

Input: nums = [1,2,3]
Output: [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]

Example 2:

Input: nums = [0,1]
Output: [[0,1],[1,0]]

Example 3:

Input: nums = [1]
Output: [[1]]

BackTracking

回溯法:

  1. 修改当前值
  2. 递归current node
  3. 还原当前值
def permute(self, nums):
“””
:type nums: List[int]
:rtype: List[List[int]]
“””

def backTrack(nums, level):
if level == len(nums)-1:
self.ans.append(list(nums))
return
for i in range(level, len(nums)):
nums[level], nums[i] = nums[i], nums[level]
backTrack(nums, level+1)
nums[level], nums[i] = nums[i], nums[level]
self.ans = []
backTrack(nums, 0)
return self.ans

Given an m x n matrix of non-negative integers representing the height of each unit cell in a continent, the "Pacific ocean" touches the left and top edges of the matrix and the "Atlantic ocean" touches the right and bottom edges.

Water can only flow in four directions (up, down…

There are n cities. Some of them are connected, while some are not. If city a is connected directly with city b, and city b is connected directly with city c, then city a is connected indirectly with city c.

A province is a group of directly or indirectly connected…

Given a non-empty 2D array grid of 0's and 1's, an island is a group of 1's (representing land) connected 4-directionally (horizontal or vertical.) You may assume all four edges of the grid are surrounded by water.

Find the maximum area of an island in the given 2D array. …

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