LeetCode in Net
128. Longest Consecutive Sequence
Medium
Given an unsorted array of integers nums, return the length of the longest consecutive elements sequence.
You must write an algorithm that runs in O(n) time.
Example 1:
Input: nums = [100,4,200,1,3,2]
Output: 4
Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.
Example 2:
Input: nums = [0,3,7,2,5,8,4,6,0,1]
Output: 9
Constraints:
0 <= nums.length <= 105-109 <= nums[i] <= 109
Solution
public class Solution {
public int LongestConsecutive(int[] nums) {
Dictionary<int, int> mapToHighest = new(nums.Length);
int best = 0;
for (int i = 0; i < nums.Length; i++) {
int rangeLow = 0;
int rangeHigh = 0;
if (mapToHighest.ContainsKey(nums[i])) {
continue;
}
if (mapToHighest.TryGetValue(nums[i]-1, out var downCount)) {
rangeLow = downCount;
}
if (mapToHighest.TryGetValue(nums[i]+1, out var upCount)) {
rangeHigh = upCount;
}
int thisSum = rangeLow + rangeHigh + 1;
mapToHighest[nums[i] - rangeLow] = thisSum;
mapToHighest[nums[i] + rangeHigh] = thisSum;
if (rangeLow != 0 && rangeHigh != 0) {
mapToHighest[nums[i]] = 1;
}
best = Math.Max(thisSum, best);
}
return best;
}
}