0969. Pancake Sorting

# 969. Pancake Sorting#

## 题目 #

Given an array A, we can perform a pancake flip: We choose some positive integer k <= A.length, then reverse the order of the first k elements of A. We want to perform zero or more pancake flips (doing them one after another in succession) to sort the array A.

Return the k-values corresponding to a sequence of pancake flips that sort A. Any valid answer that sorts the array within 10 * A.length flips will be judged as correct.

Example 1:

``````
Input: [3,2,4,1]
Output: [4,2,4,3]
Explanation:
We perform 4 pancake flips, with k values 4, 2, 4, and 3.
Starting state: A = [3, 2, 4, 1]
After 1st flip (k=4): A = [1, 4, 2, 3]
After 2nd flip (k=2): A = [4, 1, 2, 3]
After 3rd flip (k=4): A = [3, 2, 1, 4]
After 4th flip (k=3): A = [1, 2, 3, 4], which is sorted.

``````

Example 2:

``````
Input: [1,2,3]
Output: []
Explanation: The input is already sorted, so there is no need to flip anything.
Note that other answers, such as [3, 3], would also be accepted.

``````

Note:

• 1 <= A.length <= 100
• A[i] is a permutation of [1, 2, …, A.length]

## 代码 #

``````
package leetcode

func pancakeSort(A []int) []int {
if len(A) == 0 {
return []int{}
}
right := len(A)
var (
ans []int
)
for right > 0 {
idx := find(A, right)
if idx != right-1 {
reverse969(A, 0, idx)
reverse969(A, 0, right-1)
ans = append(ans, idx+1, right)
}
right--
}

return ans
}

func reverse969(nums []int, l, r int) {
for l < r {
nums[l], nums[r] = nums[r], nums[l]
l++
r--
}
}

func find(nums []int, t int) int {
for i, num := range nums {
if num == t {
return i
}
}
return -1
}

``````

Apr 8, 2023