Search for the k-th largest number in two sorted order arrays (one increasing and one decreasing)

The question is not difficult. The key is to clarify the boundary conditions. First, write a solution with the time complexity of O (k.

#include <iostream>using namespace std;//a[] increase//b[] decrease//use ret_value to return the result//function ret reprsent the error if not 0int find_k(int a[], int b[], int m, int n, int k, int& ret_value) {    if(k<=0 || m<0 || n<0 || k>(m+n)) {        return -1;    }    int i = 0, x = m-1, y = 0;    while (i<k && x>=0 && y<=n-1) {        if (a[x] > b[y]) {            ret_value = a[x];        i++;        x--;    } else if (a[x] < b[y]) {            ret_value = b[y];        i++;            y++;    } else { //equal            ret_value = a[x];        i+=2;        x--;        y++;    }    }    cout << "x:" << x << endl          << "y:" << y << endl         << "i:" << i << endl         << "v:" << ret_value << endl;    i = k-i;    if (i > 0) {        if (x<0) {            ret_value = a[y+i-1];        } else if (y > n-1) {            ret_value = a[x-i+1];        }    }    return 0;}int main () {   int a[] = {1,2,3};   int b[] = {6,5,3};   int k=0;   cout << "please input the k:";   cin >> k;   int value = -1;   find_k(a,b,3,3,k,value);   cout << value << endl;      return 0;}

Attach an algorithm implementation with the complex time server O (log n,

The algorithm IDEA is:

Each time we compare the number of K/2 in two arrays, and the number of K/2 in the array where the larger number is located, it must be in the top K number;

In this way, the number k/2 is excluded, and then the number k/2 is found in the remaining number, until the last number is found.

 

To simplify the process, we assume that both arrays are in descending order.

Note the 31 ~ 34 rows. If they are not judged to be equal, when there is one remaining number, 20th rows

mid_b = pb + kb -1;

Mid_ B will be assigned a wrong position, leading to an endless loop of programs.

//both a[] b[] decreaseint find_k_both_increase_O_log_k(int a[], int b[], int m, int n, int k, int& ret_value) {    if(k<=0 || m<0 || n<0 || k>(m+n)) {        return -1;    }    int ka = 0;    int kb = 0;    int pa = 0;    int pb = 0;    int mid_a = 0;    int mid_b = 0;    int i = k;    while (i>0 && pa<m && pb<n) {        ka = i/2;        kb = i-ka;        mid_a = pa + ka -1;        mid_b = pb + kb -1;        if (mid_a<m && mid_b<n) {            if (a[mid_a] > b[mid_b]) {                ret_value = a[mid_a];                pa = mid_a + 1;                i = i - ka;            } else if (a[mid_a] < b[mid_b]) {                ret_value = b[mid_b];                pb = mid_b + 1;                i = i - kb;            } else { //equal                ret_value = a[mid_a];                return 0;            }        }                cout << "pa:" << pa << endl             << "pb:" << pb << endl             << "i:" << i << endl;        cout <<endl;    }    if (i > 0) {        if (pa > m-1) {            ret_value = a[pb-1+i];        } else if (pb > n-1) {            ret_value = a[pa-1+i];        }    }    return 0;}

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Search for the k-th largest number in two sorted order arrays (one increasing and one decreasing)