# 969. Pancake Sorting

Given an array of integers `arr`, sort the array by performing a series of pancake flips.

In one pancake flip we do the following steps:

• Choose an integer `k` where `1 <= k <= arr.length`.
• Reverse the sub-array `arr[1...k]`.

For example, if `arr = [3,2,1,4]` and we performed a pancake flip choosing `k = 3`, we reverse the sub-array `[3,2,1]`, so `arr = [1,2,3,4]` after the pancake flip at `k = 3`.

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

Example 1:

``````Input: arr = [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: arr = [3, 2, 4, 1]
After 1st flip (k = 4): arr = [1, 4, 2, 3]
After 2nd flip (k = 2): arr = [4, 1, 2, 3]
After 3rd flip (k = 4): arr = [3, 2, 1, 4]
After 4th flip (k = 3): arr = [1, 2, 3, 4], which is sorted.
Notice that we return an array of the chosen k values of the pancake flips.
``````

Example 2:

``````Input: arr = [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.
``````

Constraints:

• `1 <= arr.length <= 100`
• `1 <= arr[i] <= arr.length`
• All integers in `arr` are unique (i.e. `arr` is a permutation of the integers from `1` to `arr.length`).

``````class Solution {
public:
vector<int> pancakeSort(vector<int>& arr) {
vector<int> res;
while (arr.size() > 1) {
int n = arr.size(), i = 0;
for (; i < n; ++i) {
if (arr[i] == n) break;
}
res.push_back(i + 1);
reverse(arr.begin(), arr.begin() + i + 1);
res.push_back(n);
reverse(arr.begin(), arr.end());
arr.pop_back();
}
return res;
}
};
``````

``````class Solution {
public:
vector<int> pancakeSort(vector<int>& arr) {
vector<int> res;
for (int i = arr.size(), j; i > 0; --i) {
for (j = 0; arr[j] != i; ++j);
reverse(arr.begin(), arr.begin() + j + 1);
res.push_back(j + 1);
reverse(arr.begin(), arr.begin() + i);
res.push_back(i);
}
return res;
}
};
``````

Github 同步地址:

https://github.com/grandyang/leetcode/issues/969

https://leetcode.com/problems/pancake-sorting/

https://leetcode.com/problems/pancake-sorting/discuss/214213/JavaC%2B%2BPython-Straight-Forward

https://leetcode.com/problems/pancake-sorting/discuss/214200/Java-flip-the-largest-number-to-the-tail

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