# Interval List Intersections Problem

## Description

LeetCode Problem 986.

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:

``````1
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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:

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

Example 3:

``````1
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Input: firstList = [], secondList = [[4,8],[10,12]]
Output: []
``````

Example 4:

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

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

## Sample C++ Code

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class Solution {
public:
vector<vector<int>> intervalIntersection(vector<vector<int>>& A, vector<vector<int>>& B) {
vector<vector<int>> res;
for (int i = 0, j = 0; i < A.size() && j < B.size();) {
int lo = max(A[i][0], B[j][0]);
int hi = min(A[i][1], B[j][1]);
if (lo <= hi)
res.push_back({lo, hi});
if (hi == A[i][1])
i++;
else
j++;
}
return res;
}
};
``````