[LeetCode] Wiggle Sort II 摆动排序之二

Given an unsorted array nums, reorder it such that nums[0] < nums[1] > nums[2] < nums[3]....

Example 1:

Input: nums = [1, 5, 1, 1, 6, 4]
Output: One possible answer is [1, 4, 1, 5, 1, 6].

Example 2:

Input: nums = [1, 3, 2, 2, 3, 1]
Output: One possible answer is [2, 3, 1, 3, 1, 2].

Note:
You may assume all input has valid answer.

Follow Up:
Can you do it in O(n) time and/or in-place with O(1) extra space?

这道题给了我们一个无序数组,让我们排序成摆动数组,满足nums[0] < nums[1] > nums[2] < nums[3]...,并给了我们例子。我们可以先给数组排序,然后在做调整。调整的方法是找到数组的中间的数,相当于把有序数组从中间分成两部分,然后从前半段的末尾取一个,在从后半的末尾去一个,这样保证了第一个数小于第二个数,然后从前半段取倒数第二个,从后半段取倒数第二个,这保证了第二个数大于第三个数,且第三个数小于第四个数,以此类推直至都取完,参见代码如下:

解法一:

// O(n) space
class Solution {
public:
void wiggleSort(vector<int>& nums) {
vector<int> tmp = nums;
int n = nums.size(), k = (n + ) / , j = n;
sort(tmp.begin(), tmp.end());
for (int i = ; i < n; ++i) {
nums[i] = i & ? tmp[--j] : tmp[--k];
}
}
};

这道题的Follow up让我们用O(n)的时间复杂度和O(1)的空间复杂度,这个真的比较难,参见网友的解答,(未完待续。。)

解法二:

// O(1) space
class Solution {
public:
void wiggleSort(vector<int>& nums) {
#define A(i) nums[(1 + 2 * i) % (n | 1)]
int n = nums.size(), i = , j = , k = n - ;
auto midptr = nums.begin() + n / ;
nth_element(nums.begin(), midptr, nums.end());
int mid = *midptr;
while (j <= k) {
if (A(j) > mid) swap(A(i++), A(j++));
else if (A(j) < mid) swap(A(j), A(k--));
else ++j;
}
}
};

类似题目:

Sort Colors

Kth Largest Element in an Array

Wiggle Sort

参考资料:

https://leetcode.com/problemset/algorithms/

https://leetcode.com/problems/wiggle-sort-ii/discuss/77706/Short-simple-C%2B%2B

https://leetcode.com/problems/wiggle-sort-ii/discuss/77677/O(n)%2BO(1)-after-median-Virtual-Indexing

https://leetcode.com/problems/wiggle-sort-ii/discuss/77682/Step-by-step-explanation-of-index-mapping-in-Java

https://leetcode.com/problems/wiggle-sort-ii/discuss/77681/O(n)-time-O(1)-space-solution-with-detail-explanations

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