動機

原來這題是greedy阿

Problem

You are given two lists of closed intervals, firstList and secondList, where firstList[i] = [starti, endi] and secondList[j] = [startj, endj]. Each list of intervals is pairwise disjoint and in sorted order.

Return the intersection of these two interval lists.

A closed interval [a, b] (with a < b) denotes the set of real numbers x with a <= x <= b.

The intersection of two closed intervals is a set of real numbers that are either empty or represented as a closed interval. For example, the intersection of [1, 3] and [2, 4] is [2, 3].

 

Example 1:

Input: firstList = [[0,2],[5,10],[13,23],[24,25]], secondList = [[1,5],[8,12],[15,24],[25,26]]Output: [[1,2],[5,5],[8,10],[15,23],[24,24],[25,25]]

Example 2:

Input: firstList = [[1,3],[5,9]], secondList = []Output: []

Example 3:

Input: firstList = [], secondList = [[4,8],[10,12]]Output: []

Example 4:

Input: firstList = [[1,7]], secondList = [[3,10]]Output: [[3,7]]

 

Constraints:

  • 0 <= firstList.length, secondList.length <= 1000
  • firstList.length + secondList.length >= 1
  • 0 <= starti < endi <= 109
  • endi < starti+1
  • 0 <= startj < endj <= 109
  • endj < startj+1

Sol

當成interval merge

class Solution:
    def intervalIntersection(self, f1: List[List[int]], f2: List[List[int]]) -> List[List[int]]:
        if f1 and f2:
            if f1[0][0] > f2[0][0]:
                return self.intervalIntersection(f2,f1)
            elif f1[0][1] < f2[0][1]:
                if f1[0][1] < f2[0][0]:
                    return self.intervalIntersection(f1[1:],f2)
                else:
                    return [[f2[0][0], f1[0][1]]] + self.intervalIntersection(f1[1:],f2)
            elif f1[0][1] > f2[0][1]:
                return [f2[0]] + self.intervalIntersection(f1,f2[1:])
            else: # ==
                return [[max(f1[0][0],f2[0][0]), f1[0][1]]] + self.intervalIntersection(f1[1:],f2)
        else:
            return []

但其實只要看兩個interval的最高最低，在最高最低的值怪怪的就不管

class Solution:
    def intervalIntersection(self, firstList: List[List[int]], secondList: List[List[int]]) -> List[List[int]]:
        A = firstList
        B = secondList
        ans = []
        i = j = 0

        while i < len(A) and j < len(B):
            # Let's check if A[i] intersects B[j].
            # lo - the startpoint of the intersection
            # hi - the endpoint of the intersection
            lo = max(A[i][0], B[j][0])
            hi = min(A[i][1], B[j][1])
            if lo <= hi:
                ans.append([lo, hi])

            # Remove the interval with the smallest endpoint
            if A[i][1] < B[j][1]:
                i += 1
            else:
                j += 1

        return ans