尾递归教学

rec.c

#include <stdio.h>

/** 求和 */

int sum_for(int i, int j) {
    int k;
    int sum = 0;
    for(k = i; k <= j; k++) {
        sum = sum + k;
    }

    return sum;
}

int sum_while(int i, int j) {
    int k = i;
    int sum = 0;

    while(1) {
        if (k == j) { 
            sum = sum + k; 
            break;
        } else {
            sum = sum + k;
            k = k + 1;
        }
    }
    
    return sum;
}

int sum_rec(int i, int j, int k) {
    int sum = 0;

    if (k == j) { 
        sum = sum + k;
    } else {
        sum = sum_rec(i, j, k + 1) + k;
    }
    return sum;
}

int sum_tailrec(int i, int j, int sum) {
    if (i == j) { 
        return sum + i;
    } else {
        return sum_tailrec(i + 1, j, sum);
    }
}

int sum_wtf(int i, int j, int sum) {
    return i == j ? sum + i : sum_wtf(i + 1, j, sum + i);
}

int main(int argc, char *argv[]) {
    printf("sum: %d\n", sum_for(1, 100));
    printf("sum: %d\n", sum_while(1, 100));
    printf("sum: %d\n", sum_rec(1, 100, 1));
    printf("sum: %d\n", sum_tailrec(1, 100, 0));
    printf("sum: %d\n", sum_wtf(1, 100, 0));
}