[Leetcode]640. Solve the Equation

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Solve a given equation and return the value of x in the form of string "x=#value". The equation contains only '+', '-' operation, the variable x and its coefficient.
If there is no solution for the equation, return "No solution".
If there are infinite solutions for the equation, return "Infinite solutions".
If there is exactly one solution for the equation, we ensure that the value of x is an integer.
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[Leetcode]1566. Detect Pattern of Length M Repeated K or More Times

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Given an array of positive integers arr,  find a pattern of length m that is repeated k or more times.
A pattern is a subarray (consecutive sub-sequence) that consists of one or more values, repeated multiple times consecutively without overlapping. A pattern is defined by its length and the number of repetitions.
Return true if there exists a pattern of length m that is repeated k or more times, otherwise return false.
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[Leetcode]621. Task Scheduler

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You are given a char array representing tasks CPU need to do. It contains capital letters A to Z where each letter represents a different task. Tasks could be done without the original order of the array. Each task is done in one unit of time. For each unit of time, the CPU could complete either one task or just be idle.
However, there is a non-negative integer n that represents the cooldown period between two same tasks (the same letter in the array), that is that there must be at least n units of time between any two same tasks.
You need to return the least number of units of times that the CPU will take to finish all the given tasks.
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[Leetcode]1513. Number of Substrings With Only 1s

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Given a binary string s (a string consisting only of '0' and '1's).
Return the number of substrings with all characters 1's.
Since the answer may be too large, return it modulo 10^9 + 7.
 
Example 1:
Input: s = "0110111" Output: 9 Explanation: There are 9 substring in total with only 1's characters. "1" -> 5 times. "11" -> 3 times. "111" -> 1 time.
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