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Search in a Sorted Array of Unknown Size in C++
Suppose we have an array and that is sorted in ascending order, we have to define a function to search target in nums. If the target is present, then return its index, otherwise, return -1.
The array size is not known. We can only access the array using an ArrayReader interface. There is a get function like ArrayReader.get(k) this will return the element of the array at index k.
So, if the input is like array = [-1,0,3,5,9,12], target = 9, then the output will be 4 as 9 exists in nums and its index is 4
To solve this, we will follow these steps −
high := 1, low := 0
while get(high) of reader < target, do −
low := high
high = high * 2
while low <= high, do −
mid := low + (high - low) / 2
x := get(mid) of reader
if x is same as target, then −
return mid
if x > target, then −
high := mid - 1
Otherwise
low := mid + 1
return -1
Example
Let us see the following implementation to get better understanding −
#include <bits/stdc++.h>
using namespace std;
class ArrayReader{
private:
vector<int> v;
public:
ArrayReader(vector<int> &v){
this->v = v;
}
int get(int i){
return v[i];
}
};
class Solution {
public:
int search(ArrayReader& reader, int target) {
int high = 1;
int low = 0;
while (reader.get(high) < target) {
low = high;
high <<= 1;
}
while (low <= high) {
int mid = low + (high - low) / 2;
int x = reader.get(mid);
if (x == target)
return mid;
if (x > target) {
high = mid - 1;
}
else
low = mid + 1;
}
return -1;
}
};
main(){
Solution ob;
vector<int> v = {-1,0,3,5,9,12};
ArrayReader reader(v);
cout<<(ob.search(reader, 9));
}Input
{-1,0,3,5,9,12}, 9Output
4
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