LeetCode 969. Pancake Sorting

原题链接在这里：https://leetcode.com/problems/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. 1 <= A.length <= 100
2. A[i] is a permutation of [1, 2, ..., A.length]

题解：

Find the max index up to largest.

swap [0, max index]

swap [0, largest]

repeat n times.

Time Complexity: O(n^2). n = A.length.

Space: O(1).

AC Java:

class Solution {
    public List<Integer> pancakeSort(int[] A) {
        List<Integer> res = new ArrayList<>();
        if(A == null){
            return res;
        }
        
        int n = A.length;
        int largest = n;
        for(int i = 0; i<n; i++){
            int maxIndex = -1;
            int max = Integer.MIN_VALUE;
            
            for(int j = 0; j<largest; j++){
                if(A[j] >= max){
                    maxIndex = j;
                    max = A[j];
                }    
            }
            
            swap(A, maxIndex);
            swap(A, largest-1);
            res.add(maxIndex + 1);
            res.add(largest--);
        }
        
        return res;
    }
    
    private void swap(int [] arr, int k){
        for(int i = 0, j = k; i<j; i++, j--){
            int temp = arr[i];
            arr[i] = arr[j];
            arr[j] = temp;
        }
    }
}