# Split Array Into Consecutive Subsequences Problem

## Description

LeetCode Problem 659.

You are given an integer array nums that is sorted in non-decreasing order.

Determine if it is possible to split nums into one or more subsequences such that both of the following conditions are true:

- Each subsequence is a consecutive increasing sequence (i.e. each integer is exactly one more than the previous integer).
- All subsequences have a length of 3 or more.

Return true if you can split nums according to the above conditions, or false otherwise.

A subsequence of an array is a new array that is formed from the original array by deleting some (can be none) of the elements without disturbing the relative positions of the remaining elements. (i.e., [1,3,5] is a subsequence of [1,2,3,4,5] while [1,3,2] is not).

Example 1:

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5

Input: nums = [1,2,3,3,4,5]
Output: true
Explanation: nums can be split into the following subsequences:
[1,2,3,3,4,5] --> 1, 2, 3
[1,2,3,3,4,5] --> 3, 4, 5

Example 2:

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5

Input: nums = [1,2,3,3,4,4,5,5]
Output: true
Explanation: nums can be split into the following subsequences:
[1,2,3,3,4,4,5,5] --> 1, 2, 3, 4, 5
[1,2,3,3,4,4,5,5] --> 3, 4, 5

Example 3:

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3

Input: nums = [1,2,3,4,4,5]
Output: false
Explanation: It is impossible to split nums into consecutive increasing subsequences of length 3 or more.

Constraints:

- 1 <= nums.length <= 10^4
- -1000 <= nums[i] <= 1000
- nums is sorted in non-decreasing order.

## Sample C++ Code

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class Solution {
public:
bool isPossible(vector<int>& A) {
unordered_map<int, int> left, end;
for (int i: A) {
left[i]++;
}
for (int i: A) {
if (left[i] == 0) continue;
left[i]--;
if (end[i-1] > 0) {
end[i-1]--;
end[i]++;
} else if (left[i+1] > 0 && left[i+2] > 0) {
left[i+1]--;
left[i+2]--;
end[i+2]++;
} else {
return false;
}
}
return true;
}
};