[daily question] 757. Set the intersection size to at least 2

greedy ：

The problem requires us to find the number of minimum point sets , The point set is not specified S It's a continuous segment .

When there is only one line segment , We can take any two points in the line segment as S member , And when there are only two segments , We can make a decision when two line segments coincide ：

• When two line segments do not coincide at all , To satisfy the meaning of the question , We need to take two points from each of the two line segments , At this time, these four points can be taken arbitrarily ;
• When only one point of two line segments coincides , To satisfy S Minimize the meaning of the question , We can take the coincidence point first , Then take one of the two line segments ;
• When two or more points of two line segments coincide , At this time, select any two of the coincidence points .

It's not hard to find out , In all cases of coincidence , It must be concluded on the edge of a certain line segment . That is, there is no coincidence point between two line segments , It only occurs in the middle of two line segments ：

Therefore, we can start from the edge point decision .
May, in accordance with the 「 The right endpoint is from small to large , The left endpoint ranges from large to small 」 Double keyword sort of , Then deal with each interval from front to back , In the process of processing, it keeps going S Add elements to it , Because we have sorted all the intervals and processed them from front to back , So we go S In the process of adding elements, it must be monotonic , At the same time, consider whether it is necessary to go to the new follow-up interval S When adding elements to meet the meaning of the question , Also with S The biggest of all / Second largest value （ Edge elements in the point set ） compare , So we can use two variables a and b Respectively refer to the current set S The second largest value and the maximum value in （a and b Initialize to a value small enough −1）, Without putting the whole S Save it .