Split Array into Consecutive Subsequences

Problem Statement - link #

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).

Examples #

Example 1:

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:

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:

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

Solutions #

class Solution {
public:
    bool isPossible(vector<int>& nums) {
        if(nums.size()<3) return false;
        unordered_map<int,int> mp;
        for(int i: nums) mp[i]++;
        unordered_map<int,int> req;
        for(int i: nums){
            if(mp[i]==0) continue;
            if(req[i]>0) {
                req[i]--;
                mp[i]--;
                req[i+1]++;
            }
            else if(mp[i]>0&&mp[i+1]>0&&mp[i+2]>0){
                mp[i]--;
                mp[i+1]--;
                mp[i+2]--;
                req[i+3]++;
            }else 
                return false;
        }
        return true;
    }
};