How to improve my c++ code's runtime efficency [closed] - c++

Question

Closed. This question does not meet Stack Overflow guidelines. It is not currently accepting answers.
This question does not appear to be about programming within the scope defined in the help center.
Closed 2 years ago.
Improve this question
I'm doing a leetcode problem 986. Interval List Intersections, I don't know why my O(m+n) solution, even the official solution only beats 5% submissions on runtime. How other people make it runs faster? Is there any ways or suggestions to improve my code? Thanks a lot.
class Solution {
public:
vector<vector<int>> intervalIntersection(vector<vector<int>>& A, vector<vector<int>>& B) {
vector<vector<int>> res;
int i = 0, j = 0;
while(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) {
vector<int> temp{lo, hi};
res.push_back(temp);
}
if(A[i][1] <= B[j][1])
++i;
else
++j;
}
return res;
}
};

Answer 1

Ok, I've done it for you. See my comments in the code.
class Solution {
public:
vector<vector<int>> intervalIntersection(vector<vector<int>>& A, vector<vector<int>>& B) {
vector<vector<int>> res;
// Avoid reallocations, reserve enough memory.
res.reserve(A.size() + B.size());
int i = 0, j = 0;
// Hint to the compiler: A.size() and B.size() are immutable.
const int ii = A.size(), jj = B.size();
while(i < ii && j < jj) {
const int lo = max(A[i][0], B[j][0]);
const int hi = min(A[i][1], B[j][1]);
if(lo <= hi) {
// Construct inner vector in-place.
res.push_back(std::vector<int>({lo, hi}));
}
if(A[i][1] <= B[j][1])
++i;
else
++j;
}
return res;
}
};
The result:
Runtime: 56 ms, faster than 46.23% of C++ online submissions for Interval List Intersections.
Memory Usage: 13 MB, less than 100.00% of C++ online submissions for Interval List Intersections.
Accepted 56 ms 13 MB cpp