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Find Right Interval in C++
Suppose we have a of intervals, for each of the interval i, check whether there exists an interval j whose start point is bigger than or equal to the endpoint of the interval i, which can be called that j is on the "right" of i. For any interval i, we have to store the minimum interval j's index, which indicates that the interval j has the minimum start point to build the "right" relationship for interval i. When the interval j doesn't exist, then store -1 for the interval i. And finally, we need output the stored value of each interval as an array. So if the input is like [[3,4], [2,3], [1,2]], so the output will be [-1, 0, 1], as there are no such right interval for [3, 4], For interval [2,3], the interval [3,4] has minimum-"right" start point; And for [1,2], the interval [2,3] has minimum-"right" start point.
To solve this, we will follow these steps −
n := size of intervals array, create am array ret of size n, and fill this using -1, crate a map called m
for i in range 0 to size of intervals
if intervals[i, 0] is in m, then skip to the next interval
m[intervals[i, 0]] := i + 1
for i in range n – 1 down to i >= 0
it := point to that key-value pair which has the smallest key, but not smaller than intervals[i, 1]
if the value of it is 0, then go for the next iteration
ret[i] := value of it – 1
return ret
Example (C++)
Let us see the following implementation to get a better understanding −
#include <bits/stdc++.h>
using namespace std;
void print_vector(vector<auto> v){
cout << "[";
for(int i = 0; i<v.size(); i++){
cout << v[i] << ", ";
}
cout << "]"<<endl;
}
class Solution {
public:
vector<int> findRightInterval(vector<vector<int>>& intervals) {
int n = intervals.size();
vector <int> ret(n, -1);
map <int, int< m;
for(int i = 0; i < intervals.size(); i++){
if(m.count(intervals[i][0])) continue;
m[intervals[i][0]] = i + 1;
}
for(int i = n - 1; i >= 0; i--){
map <int, int> :: iterator it = m.lower_bound(intervals[i][1]);
if(it->second == 0) continue;
ret[i] = it->second - 1;
}
return ret;
}
};
main(){
vector<vector<int>> v = {{3,4},{2,3},{1,2}};
Solution ob;
print_vector(ob.findRightInterval(v));
}Input
[[3,4],[2,3],[1,2]]
Output
[-1,0,1]
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