Posts published in September 2017

Problem:

Given a string containing only three types of characters: ‘(‘, ‘)’ and ‘*’, write a function to check whether this string is valid. We define the validity of a string by these rules:

1. Any left parenthesis '(' must have a corresponding right parenthesis ')'.
2. Any right parenthesis ')' must have a corresponding left parenthesis '('.
3. Left parenthesis '(' must go before the corresponding right parenthesis ')'.
4. '*' could be treated as a single right parenthesis ')' or a single left parenthesis '(' or an empty string.
5. An empty string is also valid.

Example 1:

Example 2:

Example 3:

Note:

1. The string size will be in the range [1, 100].

Idea:

Dynamic Programming / Counting

Solution 1:

C++ / DP / Top-down O(n^3)

C++ / DP/ Bottom-up O(n^3)

C++ / Counting O(n)

Java / DP / Bottom-up O(n^3)

Problem:

There is a garden with N slots. In each slot, there is a flower. The N flowers will bloom one by one in N days. In each day, there will be exactly one flower blooming and it will be in the status of blooming since then.

Given an array flowers consists of number from 1 to N. Each number in the array represents the place where the flower will open in that day.

For example, flowers[i] = x means that the unique flower that blooms at day i will be at position x, where i and x will be in the range from 1 to N.

Also given an integer k, you need to output in which day there exists two flowers in the status of blooming, and also the number of flowers between them is k and these flowers are not blooming.

If there isn’t such day, output -1.

Example 1:

Input:
flowers: [1,3,2]
k: 1
Output: 2
Explanation: In the second day, the first and the third flower have become blooming.


Example 2:

Input:
flowers: [1,2,3]
k: 1
Output: -1


Note:

1. The given array will be in the range [1, 20000].

Idea:

BST/Buckets

Solution 1: TLE with latest test cases

Python

https://leetcode.com/problems/employee-importance/description/

Problem:

ou are given a data structure of employee information, which includes the employee’s unique id, his importance value and his direct subordinates’ id.

For example, employee 1 is the leader of employee 2, and employee 2 is the leader of employee 3. They have importance value 15, 10 and 5, respectively. Then employee 1 has a data structure like [1, 15, [2]], and employee 2 has [2, 10, [3]], and employee 3 has [3, 5, []]. Note that although employee 3 is also a subordinate of employee 1, the relationship is not direct.

Now given the employee information of a company, and an employee id, you need to return the total importance value of this employee and all his subordinates.

Example 1:

Note:

1. One employee has at most one direct leader and may have several subordinates.
2. The maximum number of employees won’t exceed 2000.

Idea:

BFS / DFS

Time complexity: O(n)

Space complexity: O(n)

Solution:

C++ / BFS

C++ / DFS

https://leetcode.com/problems/redundant-connection/description/

Problem:

In this problem, a tree is an undirected graph that is connected and has no cycles.

The given input is a graph that started as a tree with N nodes (with distinct values 1, 2, …, N), with one additional edge added. The added edge has two different vertices chosen from 1 to N, and was not an edge that already existed.

The resulting graph is given as a 2D-array of edges. Each element of edges is a pair [u, v] with u < v, that represents an undirected edge connecting nodes u and v.

Return an edge that can be removed so that the resulting graph is a tree of N nodes. If there are multiple answers, return the answer that occurs last in the given 2D-array. The answer edge [u, v] should be in the same format, with u < v.

Example 1:

Example 2:

Note:

• The size of the input 2D-array will be between 3 and 1000.
• Every integer represented in the 2D-array will be between 1 and N, where N is the size of the input array.

Idea:
DFS / Union-Find

Solutions:

C++ / DFS

C++ / Union Find

Java / Union Find

Python: Union Find

Python / Union Find V2

Problem:

Implement a trie with insertsearch, and startsWith methods.

Note:
You may assume that all inputs are consist of lowercase letters a-z.

Idea:

Tree/children array

Solution:

C++ / Array

C++ / hashmap

Java

Python 1:

Python 2:

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