Press "Enter" to skip to content

Posts tagged as “medium”

花花酱 LeetCode 1043. Partition Array for Maximum Sum

Given an integer array A, you partition the array into (contiguous) subarrays of length at most K.  After partitioning, each subarray has their values changed to become the maximum value of that subarray.

Return the largest sum of the given array after partitioning.

Example 1:

Input: A = [1,15,7,9,2,5,10], K = 3
Output: 84
Explanation: A becomes [15,15,15,9,10,10,10]

Note:

  1. 1 <= K <= A.length <= 500
  2. 0 <= A[i] <= 10^6

Solution: DP

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

dp[i] := max sum of A[1] ~ A[i]
init: dp[0] = 0
transition: dp[i] = max{dp[i – k] + max(A[i-k:i]) * k}, 1 <= k <= min(i, K)
ans: dp[n]

A = | 2 | 1 | 4 | 3 |
K = 3
dp[0] = 0
dp[1] = max(dp[0] + 2 * 1) = 2
dp[2] = max(dp[0] + 2 * 2, dp[1] + 1 * 1) = max(4, 3) = 4
dp[3] = max(dp[0] + 4 * 3, dp[1] + 4 * 2, dp[2] + 4 * 1) = max(12, 10, 8) = 12
dp[4] = max(dp[1] + 4 * 3, dp[2] + 4 * 2, dp[3] + 3 * 1) = max(14, 12, 15) = 15
best = | 4 | 4 | 4 | 3 |

C++

花花酱 LeetCode 640. Solve the Equation

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.

Example 1:

Input: "x+5-3+x=6+x-2"
Output: "x=2"

Example 2:

Input: "x=x"
Output: "Infinite solutions"

Example 3:

Input: "2x=x"
Output: "x=0"

Example 4:

Input: "2x+3x-6x=x+2"
Output: "x=-1"

Example 5:

Input: "x=x+2"
Output: "No solution"

Solution: Parse the equation

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

C++

花花酱 LeetCode 838. Push Dominoes

here are N dominoes in a line, and we place each domino vertically upright.

In the beginning, we simultaneously push some of the dominoes either to the left or to the right.

After each second, each domino that is falling to the left pushes the adjacent domino on the left.

Similarly, the dominoes falling to the right push their adjacent dominoes standing on the right.

When a vertical domino has dominoes falling on it from both sides, it stays still due to the balance of the forces.

For the purposes of this question, we will consider that a falling domino expends no additional force to a falling or already fallen domino.

Given a string “S” representing the initial state. S[i] = 'L', if the i-th domino has been pushed to the left; S[i] = 'R', if the i-th domino has been pushed to the right; S[i] = '.', if the i-th domino has not been pushed.

Return a string representing the final state. 

Example 1:

Input: ".L.R...LR..L.."
Output: "LL.RR.LLRRLL.."

Example 2:

Input: "RR.L"
Output: "RR.L"
Explanation: The first domino expends no additional force on the second domino.

Note:

  1. 0 <= N <= 10^5
  2. String dominoes contains only 'L‘, 'R' and '.'

Solution: Simulation

Simulate the push process, record the steps from L and R for each domino.
steps(L) == steps(R) => “.”
steps(L) < steps(R) => “L”
steps(L) > steps(R) => “R”

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

C++

花花酱 LeetCode 279. Perfect Squares

Given a positive integer n, find the least number of perfect square numbers (for example, 1, 4, 9, 16, ...) which sum to n.

Example 1:

Input: n = 12
Output: 3 
Explanation: 12 = 4 + 4 + 4.

Example 2:

Input: n = 13
Output: 2
Explanation: 13 = 4 + 9.

Solution 1: DP

dp[i] := ans
dp[0] = 0
dp[i] = min{dp[i – j * j] + 1} 1 <= j * j <= i

dp[5] = min{
dp[5 – 2 * 2] + 1 = dp[1] + 1 = (dp[1 – 1 * 1] + 1) + 1 = dp[0] + 1 + 1 = 2,
dp[5 – 1 * 1] + 1 = dp[3] + 1 = (dp[3 – 1 * 1] + 1) + 1 = dp[1] + 2 = dp[1 – 1*1] + 1 + 2 = dp[0] + 3 = 3
};

dp[5] = 2

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

C++

花花酱 LeetCode 54. Spiral Matrix

Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

Example 1:

Input:
[
 [ 1, 2, 3 ],
 [ 4, 5, 6 ],
 [ 7, 8, 9 ]
]
Output: [1,2,3,6,9,8,7,4,5]

Example 2:

Input:
[
  [1, 2, 3, 4],
  [5, 6, 7, 8],
  [9,10,11,12]
]
Output: [1,2,3,4,8,12,11,10,9,5,6,7]

Solution: Simulation

Keep track of the current bounds (left, right, top, bottom).

Init: left = 0, right = n – 1, top = 0, bottom = m – 1

Each time we move in one direction and shrink the bounds and turn 90 degrees:
1. go right => –top
2. go down => –right
3. go left => ++bottom
4. go up => ++left

C++