646. Maximum Length of Pair Chain

1. Description

You are given an array of n pairs pairs where pairs[i] = [$left_i$, $right_i$] and $left_i$ < $right_i$.
A pair p2 = [c, d] follows a pair p1 = [a, b] if b < c. A chain of pairs can be formed in this fashion.
Return the length longest chain which can be formed.
You do not need to use up all the given intervals. You can select pairs in any order.

2. Example

Example 1:
Input: pairs = [[1,2],[2,3],[3,4]]
Output: 2
Explanation: The longest chain is [1,2] -> [3,4].

Example 2:
Input: pairs = [[1,2],[7,8],[4,5]]
Output: 3
Explanation: The longest chain is [1,2] -> [4,5] -> [7,8].

3. Constraints

• n == pairs.length
• 1 <= n <= 1000
• -1000 <= $left_i$ < $right_i$ <= 1000

4. Solutions

Greedy

n = pairs.size()
Time complexity: O(nlogn)
Space complexity: O(logn)

class Solution {
public:
    int findLongestChain(vector<vector<int>> pairs) {
        sort(pairs.begin(), pairs.end(), [](const vector<int> &pair1, const vector<int> &pair2) {
            return pair1[1] < pair2[1];
        });

        int result = 0, last = INT_MIN;
        for (auto nums : pairs) {
            if (nums[0] > last) {
                last = nums[1];
                ++result;
            }
        }

        return result;
    }
};