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Posts tagged as “combination”

花花酱 LeetCode 22. Generate Parentheses

Problem

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

[
  "((()))",
  "(()())",
  "(())()",
  "()(())",
  "()()()"
]

Solution: DFS

Time complexity: O(2^n)

Space complexity: O(k + n)

C++

Related Problems

花花酱 LeetCode 491. Increasing Subsequences

Problem

Given an integer array, your task is to find all the different possible increasing subsequences of the given array, and the length of an increasing subsequence should be at least 2 .

Example:

Input: [4, 6, 7, 7]
Output: [[4, 6], [4, 7], [4, 6, 7], [4, 6, 7, 7], [6, 7], [6, 7, 7], [7,7], [4,7,7]]

Note:

  1. The length of the given array will not exceed 15.
  2. The range of integer in the given array is [-100,100].
  3. The given array may contain duplicates, and two equal integers should also be considered as a special case of increasing sequence.

Solution: DFS

Time complexity: O(2^n)

Space complexity: O(n)

C++

 

花花酱 LeetCode 401. Binary Watch

Problem:

A binary watch has 4 LEDs on the top which represent the hours (0-11), and the 6 LEDs on the bottom represent the minutes (0-59).

Each LED represents a zero or one, with the least significant bit on the right.

For example, the above binary watch reads “3:25”.

Given a non-negative integer n which represents the number of LEDs that are currently on, return all possible times the watch could represent.

Example:

Input: n = 1
Return: ["1:00", "2:00", "4:00", "8:00", "0:01", "0:02", "0:04", "0:08", "0:16", "0:32"]

Note:

  • The order of output does not matter.
  • The hour must not contain a leading zero, for example “01:00” is not valid, it should be “1:00”.
  • The minute must be consist of two digits and may contain a leading zero, for example “10:2” is not valid, it should be “10:02”.

Solution 1:

Time complexity: O(11*59*n)

C++

 

花花酱 LeetCode 494. Target Sum

题目大意:给你一串数字,你可以在每个数字前放置+或-,问有多少种方法可以使得表达式的值等于target。You are given a list of non-negative integers, a1, a2, …, an, and a target, S. Now you have 2 symbols + and -. For each integer, you should choose one from + and - as its new symbol.

Find out how many ways to assign symbols to make sum of integers equal to target S.

Example 1:

Note:

  1. The length of the given array is positive and will not exceed 20.
  2. The sum of elements in the given array will not exceed 1000.
  3. Your output answer is guaranteed to be fitted in a 32-bit integer.

 

Idea: DP


Solution 1: DP

Time complexity: O(n*sum)

Space complexity: O(n*sum)

C++

C++ SC O(n)

Java

C++ / V2

Solution 2: DFS

Time complexity: O(2^n)

Space complexity: O(n)

C++

Java

Solution 3: Subset sum

Time complexity: O(n*sum)

Space complexity: O(sum)

C++ w/ copy

C++ w/o copy

花花酱 LeetCode 17. Letter Combinations of a Phone Number

题目大意:给你一串电话号码,输出可以由这串电话号码打出的所有字符串。

Problem:

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

Solution 1: DFS

C++

Java

Python3

Solution 2:  BFS

C++

Java

Python