65 lines
1.6 KiB
C++
65 lines
1.6 KiB
C++
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/*
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Description: Index of first 1 in a sorted array of 0s and 1s.
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Given a sorted array consisting 0’s and 1’s. The task is to find the index of first ‘1’ in the given array.
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Approach: Use binary search to solve in least time complexity (O log n).
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*/
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#include <iostream>
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#include <algorithm>
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using namespace std;
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int firstIndex(int a[], int n) {
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int low=0;
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int high=n-1;
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while(low<=high) {
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int mid = (low+high)/2;
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//if mid appears to be 0, update low as mid+1 index because we want to search 1 in array
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if(a[mid]==0){
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low=mid+1;
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}
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//checking if mid element is 1 and the element at mid-1 is 0
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//if it is so, then we have found index of first 1
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//simply return mid
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else if(a[mid]==1 && (mid==0 || a[mid-1]==0)){
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return mid;
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}
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//updating the high to mid-1
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//because the element at mid-1 index is not zero
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else if (a[mid]==1){
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high=mid-1;
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}
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}
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//if index not found, return -1
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return -1;
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}
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int main() {
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int n;
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cout << "Enter number of elements in an array\n";
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cin>>n;
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int a[n];
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cout<<"Enter array elements: (only 0 and 1 in sorted order)\n";
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for(int i=0;i<n;i++){
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cin>>a[i];
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}
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//if someone forgets to pass sorted array
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sort(a,a+n);
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int ind = firstIndex(a,n);
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cout<<"Index of first 1 in sorted array: "<<ind;
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}
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/*
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Sample Input:
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Enter number of elements in an array
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4
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Enter array elements: (only 0 and 1 in sorted order)
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0
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0
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0
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1
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Output:
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Index of first 1 in sorted array: 3
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*/
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