Array Nesting Problem
Description
LeetCode Problem 565.
You are given an integer array nums of length n where nums is a permutation of the numbers in the range [0, n - 1].
You should build a set s[k] = {nums[k], nums[nums[k]], nums[nums[nums[k]]], … } subjected to the following rule:
- The first element in s[k] starts with the selection of the element nums[k] of index = k.
- The next element in s[k] should be nums[nums[k]], and then nums[nums[nums[k]]], and so on.
- We stop adding right before a duplicate element occurs in s[k].
Return the longest length of a set s[k].
Example 1:
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Input: nums = [5,4,0,3,1,6,2]
Output: 4
Explanation:
nums[0] = 5, nums[1] = 4, nums[2] = 0, nums[3] = 3, nums[4] = 1, nums[5] = 6, nums[6] = 2.
One of the longest sets s[k]:
s[0] = {nums[0], nums[5], nums[6], nums[2]} = {5, 6, 2, 0}
Example 2:
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Input: nums = [0,1,2]
Output: 1
Constraints:
- 1 <= nums.length <= 10^5
- 0 <= nums[i] < nums.length
- All the values of nums are unique.
Sample C++ Code
The idea is to, start from every number, find circles in those index-pointer-chains. Every time we find a set (a circle) mark every number as visited (-1) so that next time we won’t step on it again.
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class Solution {
public:
int arrayNesting(vector<int>& a) {
size_t maxsize = 0;
for (int i = 0; i < a.size(); i++) {
size_t size = 0;
for (int k = i; a[k] >= 0; size++) {
int ak = a[k];
a[k] = -1; // mark a[k] as visited;
k = ak;
}
maxsize = max(maxsize, size);
}
return maxsize;
}
};