编译正常但提交后出现RUN_ERROR：数组最近重复元素程序排查

Hey there! Let's dig into why your code is hitting a RUN_ERROR when submitted. I've gone through your implementation and spotted a critical issue that's causing this runtime crash:

Your code uses a[abs(a[i])] to mark whether an element has appeared before, but this creates a severe array out-of-bounds error:

• The array a has a size of n (the user-input length of the array), but the problem allows element values up to 10^5.
• If n is smaller than any element's value (e.g., n=5 but an element is 10), accessing a[10] will reach memory outside the array's allocated space, which directly triggers a runtime error—this is exactly the RUN_ERROR you're seeing.

Example of the Issue #

Suppose the user inputs n=3 followed by elements [2, 3, 4]. When processing the third element 4, your code tries to access a[4], but the array only has indices 0, 1, and 2. This invalid memory access crashes the program.

To fix this, you need a marker array that can cover all possible element values (1 to 10^5), instead of repurposing the input array (which might be too small). Here's the corrected version:

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

void input(int *x);

int main(){
    int n;
    input(&n);
    
    // Create a boolean array to track seen elements (covers 1 to 100000)
    bool *seen = (bool*)calloc(100001, sizeof(bool));
    if (seen == NULL) { // Handle memory allocation failure
        return 1;
    }
    
    int num;
    for(int i=0;i<n;i++){
        if (scanf("%d",&num)!=1) {
            free(seen); // Clean up memory before exiting
            return 0;
        }
        if (num > 100000 || num < 1) {
            free(seen);
            return 0;
        }
        
        if (seen[num]) { // Found the first duplicate
            printf("%d", num);
            free(seen);
            return 0;
        } else {
            seen[num] = true; // Mark element as seen
        }
    }
    
    free(seen); // Release allocated memory
    return 0;
}

void input(int *x){
    if (scanf("%d",x)!=1) exit(0);
    if (*x > 100000 || *x <1) exit(0);
}

• Used calloc to initialize the marker array to false automatically, which is cleaner than manual initialization.
• Removed the need to store the entire input array—we only need to check duplicates as we read each element.
• Added memory cleanup with free() to avoid memory leaks.
• Used a bool array instead of an integer array to save memory (1 byte per element vs. 4 bytes for integers).

Your original code also had a minor logical flaw: if an element's value equals an array index, it could incorrectly mark elements as duplicate, but the out-of-bounds access was the immediate cause of the RUN_ERROR.

内容的提问来源于stack exchange，提问作者Nguyen Huu Tien K14 HCM