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This approach leverages closures to keep track of whether the original function has been called. A closure allows the function to maintain state across calls, which in this case is whether the function should execute or return undefined.
Time Complexity: O(1) for each call, Space Complexity: O(1), as no additional data structures are used.
1def once(fn):
2This Python solution utilizes an inner function with nonlocal keyword to maintain the state of called and result. On first call, the result is computed and stored, while subsequent calls return None.
In this approach, use an external flag variable to keep track of whether the function has already been executed. This approach is more language-specific where closure support is limited.
Time Complexity: O(1) per call, Space Complexity: O(1) for state flag.
1#include <stdbool.h>
2#include <stdlib.h>
3#include <stdio.h>
4
5typedef int (*Function)(int, int, int);
6
7typedef struct {
8 Function fn;
9 bool called;
10 int result;
11} Once;
12
13Once* once(Function fn) {
14 Once* o = (Once*)malloc(sizeof(Once));
15 o->fn = fn;
16 o->called = false;
17 o->result = 0;
18 return o;
19}
20
21int once_call(Once* o, int a, int b, int c) {
22 if (!o->called) {
23 o->called = true;
24 o->result = o->fn(a, b, c);
25 return o->result;
26 }
27 return -1; // Represent undefined
28}
29
30int sum(int a, int b, int c) {
31 return a + b + c;
32}
33
34int main() {
35 Once* onceFn = once(sum);
36 printf("%d\n", once_call(onceFn, 1, 2, 3)); // Output the sum
37 printf("%d\n", once_call(onceFn, 2, 3, 6)); // Output undefined equivalent
38 free(onceFn);
39 return 0;
40}
41This C solution demonstrates using a structure to maintain state. The Once struct includes a function pointer and a flag called which checks for the first invocation. once_call executes only on the first call, thereafter returns an undefined equivalent.