ternary_search.cpp

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#include <iostream>
 
#define absolutePrecision 10
#define _target 10
 
#define MAX 10000000  
 
void get_input() {}
 
int it_ternary_search(int left, int right, int A[], int target) {
    while (1) {
        if (left < right) {
            if (right - left < absolutePrecision) {
                for (int i = left; i <= right; i++)
                    if (A[i] == target)
                        return i;
 
                return -1;
            }
 
            int oneThird = (left + right) / 3 + 1;
            int twoThird = (left + right) * 2 / 3 + 1;
 
            if (A[oneThird] == target)
                return oneThird;
            else if (A[twoThird] == target)
                return twoThird;
 
            else if (target > A[twoThird])
                left = twoThird + 1;
            else if (target < A[oneThird])
                right = oneThird - 1;
 
            else
                left = oneThird + 1, right = twoThird - 1;
        } else {
            return -1;
        }
    }
}
 
int rec_ternary_search(int left, int right, int A[], int target) {
    if (left < right) {
        if (right - left < absolutePrecision) {
            for (int i = left; i <= right; i++)
                if (A[i] == target)
                    return i;
 
            return -1;
        }
 
        int oneThird = (left + right) / 3 + 1;
        int twoThird = (left + right) * 2 / 3 + 1;
 
        if (A[oneThird] == target)
            return oneThird;
        if (A[twoThird] == target)
            return twoThird;
 
        if (target < A[oneThird])
            return rec_ternary_search(left, oneThird - 1, A, target);
        if (target > A[twoThird])
            return rec_ternary_search(twoThird + 1, right, A, target);
 
        return rec_ternary_search(oneThird + 1, twoThird - 1, A, target);
    } else {
        return -1;
    }
}
 
void ternary_search(int N, int A[], int target) {
    std::cout << it_ternary_search(0, N - 1, A, target) << '\t';
    std::cout << rec_ternary_search(0, N - 1, A, target) << '\t';
    std::cout << std::endl;
}
 
int main() {
    int N = 21;
    int A[] = {1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 3, 4, 10};
    get_input();
    ternary_search(N, A, _target);
    return 0;
}