BackTracking_Fixed sum for array elements

 

Given an array a contains distinct positive integers, count how many combinations of integers in a add up to exactly sum

For example, given int[] a = {11, 3, 8} ; and sum = 11

You should output 2, because 11 == 11 and 3 + 8 == 11

This is typically a backtracking problem

Enumerate all the subsets of the given array to see how many of them match the condition

when you write backtracking procedure using recursion, please be careful of which condition do you

use to terminate the loop, in this code snippet, there two conditions,

1. sum == 0

2. t == a.Length

and when t == a.Length, we might be got an solution yet, don't forget this case.

Backtracking

 1 class FixedSum
 2 {
 3     public int Count(int[] a, int sum)
 4     {
 5         Backtrack(a, 0, sum);
 6         return count;
 7     }
 8 
 9     // t is the index used to control the recursive procedure
10     public void Backtrack(int[] a, int t, int sum)
11     {
12         if (t == a.Length)
13         {
14             // although t is out of range, but we got a solution
15             // that means the last element of a is part of solution
16             if (sum == 0)
17                 count++;
18             return;
19         }
20 
21         if (sum == 0) // got a solution
22             count++;
23         else
24         {
25             if (sum - a[t] >= 0)
26                 Backtrack(a, t + 1, sum - a[t]);// enter left tree
27 
28             if (sum >= 0)
29                 Backtrack(a, t + 1, sum); // enter right tree
30         }
31     }
32 
33     // Count how many solutions there is
34     private int count = 0;
35 
36 }
37 

recursive way

// i is the index, n is sum
public int Combo(int[] a, int i, int n)
{
    if (n == 0) // sum is 0, then no element needed, so it's an empty solution
        return 1;
    if (i < 0) // index < 0, no solution found
        return 0;
    if (a[i] > n) // exclude the element greater than n
        return Combo(a, i - 1, n);
    else // else compute the solution include n plus exclude n.
        return Combo(a, i - 1, n) + Combo(a, i - 1, n - a[i]);
}

C法

#include<stdio.h>
#include<stdlib.h>

int count = 0; // number of solutions

/*
 * array - positive numbers
 * n     - element count in array
 * sum   - pair of sum
 * t     - recursion deep
 */
void find_combinations(int *array, int n, int sum, int t) {
    if (t == n) {
        if (sum == 0) {
            count++;
        }
        return;
    }

    if (sum == 0) { // Find a solution
        count++;
    }
    else {
        if (sum >= array[t]) {  // left tree
            find_combinations(array, n, sum - array[t], t + 1);
        }
        if (sum > 0) {                  // right tree
            find_combinations(array, n, sum, t + 1);
        }
    }
}

int main(void) {
    int a[] = {11, 3, 8, 4, 1, 7};
    find_combinations(a, 6, 11, 0);
    printf("%d\n", count);

    system("pause");
    return 0;
}

==