# Posts published in “Stack”

You are given an array of integers nums. Perform the following steps:

1. Find any two adjacent numbers in nums that are non-coprime.
2. If no such numbers are found, stop the process.
3. Otherwise, delete the two numbers and replace them with their LCM (Least Common Multiple).
4. Repeat this process as long as you keep finding two adjacent non-coprime numbers.

Return the final modified array. It can be shown that replacing adjacent non-coprime numbers in any arbitrary order will lead to the same result.

The test cases are generated such that the values in the final array are less than or equal to 108.

Two values x and y are non-coprime if GCD(x, y) > 1 where GCD(x, y) is the Greatest Common Divisor of x and y.

Example 1:

Input: nums = [6,4,3,2,7,6,2]
Output: [12,7,6]
Explanation:
- (6, 4) are non-coprime with LCM(6, 4) = 12. Now, nums = [12,3,2,7,6,2].
- (12, 3) are non-coprime with LCM(12, 3) = 12. Now, nums = [12,2,7,6,2].
- (12, 2) are non-coprime with LCM(12, 2) = 12. Now, nums = [12,7,6,2].
- (6, 2) are non-coprime with LCM(6, 2) = 6. Now, nums = [12,7,6].
There are no more adjacent non-coprime numbers in nums.
Thus, the final modified array is [12,7,6].
Note that there are other ways to obtain the same resultant array.


Example 2:

Input: nums = [2,2,1,1,3,3,3]
Output: [2,1,1,3]
Explanation:
- (3, 3) are non-coprime with LCM(3, 3) = 3. Now, nums = [2,2,1,1,3,3].
- (3, 3) are non-coprime with LCM(3, 3) = 3. Now, nums = [2,2,1,1,3].
- (2, 2) are non-coprime with LCM(2, 2) = 2. Now, nums = [2,1,1,3].
There are no more adjacent non-coprime numbers in nums.
Thus, the final modified array is [2,1,1,3].
Note that there are other ways to obtain the same resultant array.


Constraints:

• 1 <= nums.length <= 105
• 1 <= nums[i] <= 105
• The test cases are generated such that the values in the final array are less than or equal to 108.

## Solution: Stack

“””It can be shown that replacing adjacent non-coprime numbers in any arbitrary order will lead to the same result.”””

So that we can do it in one pass from left to right using a stack/vector.

Push the current number onto stack, and merge top two if they are not co-prime.

Time complexity: O(nlogm)
Space complexity: O(n)

## C++

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

Implement the MinStack class:

• MinStack() initializes the stack object.
• void push(int val) pushes the element val onto the stack.
• void pop() removes the element on the top of the stack.
• int top() gets the top element of the stack.
• int getMin() retrieves the minimum element in the stack.

Example 1:

Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],,[-3],[],[],[],[]]

Output


[null,null,null,null,-3,null,0,-2]

Explanation MinStack minStack = new MinStack(); minStack.push(-2); minStack.push(0); minStack.push(-3); minStack.getMin(); // return -3 minStack.pop(); minStack.top(); // return 0 minStack.getMin(); // return -2

Constraints:

• -231 <= val <= 231 - 1
• Methods poptop and getMin operations will always be called on non-empty stacks.
• At most 3 * 104 calls will be made to pushpoptop, and getMin.

## Solution 1: Two Stacks

One normal stack, one monotonic stack to store the min values.

Time complexity: O(1) per op
Space complexity: O(n)

## C++

There are n cars traveling at different speeds in the same direction along a one-lane road. You are given an array cars of length n, where cars[i] = [positioni, speedi] represents:

• positioni is the distance between the ith car and the beginning of the road in meters. It is guaranteed that positioni < positioni+1.
• speedi is the initial speed of the ith car in meters per second.

For simplicity, cars can be considered as points moving along the number line. Two cars collide when they occupy the same position. Once a car collides with another car, they unite and form a single car fleet. The cars in the formed fleet will have the same position and the same speed, which is the initial speed of the slowest car in the fleet.

Return an array answer, where answer[i] is the time, in seconds, at which the ith car collides with the next car, or -1 if the car does not collide with the next car. Answers within 10-5 of the actual answers are accepted.

Example 1:

Input: cars = [[1,2],[2,1],[4,3],[7,2]]
Output: [1.00000,-1.00000,3.00000,-1.00000]
Explanation: After exactly one second, the first car will collide with the second car, and form a car fleet with speed 1 m/s. After exactly 3 seconds, the third car will collide with the fourth car, and form a car fleet with speed 2 m/s.


Example 2:

Input: cars = [[3,4],[5,4],[6,3],[9,1]]
Output: [2.00000,1.00000,1.50000,-1.00000]


Constraints:

• 1 <= cars.length <= 105
• 1 <= positioni, speedi <= 106
• positioni < positioni+1

## Solution: Monotonic Stack

Key observation: If my speed is slower than the speed of the previous car, not only mine but also all cars behind me will NEVER be able to catch/collide with the previous car. Such that we can throw it away.

Maintain a stack that stores the indices of cars with increasing speed.

Process car from right to left, for each car, pop the stack (throw the fastest car away) if any of the following conditions hold.
1) speed <= stack.top().speed
2) There are more than one car before me and it takes more than to collide the fastest car than time the fastest took to collide.

Time complexity: O(n)
Space complexity: O(n)

## C++

A string is a valid parentheses string (denoted VPS) if it meets one of the following:

• It is an empty string "", or a single character not equal to "(" or ")",
• It can be written as AB (A concatenated with B), where A and B are VPS‘s, or
• It can be written as (A), where A is a VPS.

We can similarly define the nesting depth depth(S) of any VPS S as follows:

• depth("") = 0
• depth(A + B) = max(depth(A), depth(B)), where A and B are VPS‘s
• depth("(" + A + ")") = 1 + depth(A), where A is a VPS.

For example, """()()", and "()(()())" are VPS‘s (with nesting depths 0, 1, and 2), and ")(" and "(()" are not VPS‘s.

Given a VPS represented as string s, return the nesting depth of s.

Example 1:

Input: s = "(1+(2*3)+((8)/4))+1"
Output: 3
Explanation: Digit 8 is inside of 3 nested parentheses in the string.


Example 2:

Input: s = "(1)+((2))+(((3)))"
Output: 3


Example 3:

Input: s = "1+(2*3)/(2-1)"
Output: 1


Example 4:

Input: s = "1"
Output: 0


Constraints:

• 1 <= s.length <= 100
• s consists of digits 0-9 and characters '+''-''*''/''(', and ')'.
• It is guaranteed that parentheses expression s is a VPS.

## Solution: Stack

We only need to deal with ‘(‘ and ‘)’

Time complexity: O(n)
Space complexity: O(1)

## C++

Given a string s of lower and upper case English letters.

A good string is a string which doesn’t have two adjacent characters s[i] and s[i + 1] where:

• 0 <= i <= s.length - 2
• s[i] is a lower-case letter and s[i + 1] is the same letter but in upper-case or vice-versa.

To make the string good, you can choose two adjacent characters that make the string bad and remove them. You can keep doing this until the string becomes good.

Return the string after making it good. The answer is guaranteed to be unique under the given constraints.

Notice that an empty string is also good.

Example 1:

Input: s = "leEeetcode"
Output: "leetcode"
Explanation: In the first step, either you choose i = 1 or i = 2, both will result "leEeetcode" to be reduced to "leetcode".


Example 2:

Input: s = "abBAcC"
Output: ""
Explanation: We have many possible scenarios, and all lead to the same answer. For example:
"abBAcC" --> "aAcC" --> "cC" --> ""
"abBAcC" --> "abBA" --> "aA" --> ""


Example 3:

Input: s = "s"
Output: "s"


Constraints:

• 1 <= s.length <= 100
• s contains only lower and upper case English letters.

## Solution: Stack

Iterator over the string, compare current char with top of the stack, if they are a bad pair, pop the stack (remove both of them). Otherwise, push the current char onto the stack.

input: “abBAcC”
“a”
“ab”
“abB” -> “a”
aA” -> “”
“c”
cC” -> “”
ans = “”

Time complexity: O(n)
Space complexity: O(n)