Minimum Points to Stab All Intervals

You are given a list of closed intervals intervals where each intervals[i] = [start, end] satisfies start < end. A point x stabs an interval [start, end] if start <= x <= end.

Return the minimum number of points such that every interval is stabbed by at least one point.

Examples

Example 1

Input:  intervals = [[10, 16],
                     [ 2,  8],
                     [ 1,  6],
                     [ 7, 12]]
Output: 2
Explanation: A point at 6 stabs [2,8] and [1,6]; a point at 11 stabs [10,16] and [7,12].

Example 2

Input:  intervals = [[1, 2],
                     [3, 4],
                     [5, 6],
                     [7, 8]]
Output: 4
Explanation: No two intervals overlap, so each one needs its own point.

Example 3

Input:  intervals = [[1, 2],
                     [2, 3],
                     [3, 4],
                     [4, 5]]
Output: 2
Explanation: A point at 2 stabs [1,2] and [2,3]; a point at 4 stabs [3,4] and [4,5].