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# 986. Interval List Intersections
双指针 区间
You are given two lists of closed intervals, `firstList` and `secondList`, where `firstList[i] = [start``i``, end``i``]` and `secondList[j] = [start``j``, end``j``]`. 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: []
**Constraints:**
* `0 <= firstList.length, secondList.length <= 1000`
* `firstList.length + secondList.length >= 1`
* `0 <= start``i`` ``< end``i`` ``<= 10``9`
* `end``i`` ``< start``i+1`
* `0 <= start``j`` ``< end``j`` ``<= 10``9`
* `end``j`` ``< start``j+1`
分析
双指针指向两区间，**移动结束时间更早的指针**。
```
class Solution:
def intervalIntersection(self, firstList: List[List[int]], secondList: List[List[int]]) -> List[List[int]]:
i = j = 0
res = []
while i < len(firstList) and j < len(secondList):
s1, e1 = firstList[i]
s2, e2 = secondList[j]
start = max(s1,s2)
end = min(e1,e2)
if start <= end:
res.append([start, end])
if e1 < e2:
i += 1
else:
j += 1
return res
```