757. Set Intersection Size At Least Two

An integer interval [a, b] (for integers a < b) is a set of all consecutive integers from a to b, including a and b.

Find the minimum size of a set S such that for every integer interval A in intervals, the intersection of S with A has size at least 2.

Example 1:

Input: intervals = [[1, 3], [1, 4], [2, 5], [3, 5]]

Output: 3

Explanation:

Consider the set S = {2, 3, 4}.  For each interval, there are at least 2 elements from S in the interval.

Also, there isn't a smaller size set that fulfills the above condition.

Thus, we output the size of this set, which is 3.

Example 2:

Input: intervals = [[1, 2], [2, 3], [2, 4], [4, 5]]

Output: 5

Explanation:

An example of a minimum sized set is {1, 2, 3, 4, 5}.

Note:

intervals will have length in range [1, 3000].
intervals[i] will have length 2, representing some integer interval.
intervals[i][j] will be an integer in [0, 10^8].

Approach #1: C++. [greedy]

class Solution {

public:

    int intersectionSizeTwo(vector<vector<int>>& intervals) {

        int size = intervals.size();

        sort(intervals.begin(), intervals.end(), [](const vector<int>& a, const vector<int>& b) {

            if (a[1] == b[1]) return a[0] > b[0];

            else return a[1] < b[1];

        });

        int ans = 0, p1 = -1, p2 = -1;

        for (int i = 0; i < size; ++i) {

            if (intervals[i][0] <= p1) continue;

            if (intervals[i][0] > p2) {

                ans += 2;

                p2 = intervals[i][1];

                p1 = p2 - 1;

            } else {

                ans++;

                p1 = p2;

                p2 = intervals[i][1];

            }

        }

        return ans;

    }

};

巴特西

757. Set Intersection Size At Least Two

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