找到一个重复元素 - 面试题

对于n个元素,每一个数据的范围[0,n-1],找到一个重复元素,空间O(1)，时间O(n)

解法一：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <iostream>
using namespace std;
int radix_sort (int *arr, int num) {
	int temp;
	for (int i = 0;i < num;i++) {
		while (i != arr[i]) {
			if (arr[i] == arr[arr[i]]) {
				return arr[i];
			}
			temp = arr[arr[i]];
			arr[arr[i]] = arr[i];
			arr[i] = temp;
		}
	}
	return -1;
}

int main (void) {
	int num;
	int arr[100];
	while (scanf ("%d", &num) != EOF) {
		for (int i = 0;i < num;i++) {
			scanf ("%d", &arr[i]);
		}
		
		int flag = radix_sort (arr, num);
		
		if (flag == -1) {
			printf ("No repeating elements\n");
		}
		else {
			printf ("Repeating elements: %d\n", flag);
		}
	}
	return 0;
}

方法二：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <iostream>
using namespace std;

int RepeatNum (int *arr, int num) {
	for (int i = 0;i < num;i++) {
		int index = arr[i] >= num ? arr[i]-num : arr[i];
		if (arr[index] >= num) {
			return index;
		}
		else {
			arr[index] += num;
		}
	}
	return -1;
}

int main (void) {
	int num;
	int arr[100];
	while (scanf ("%d", &num) != EOF) {
		for (int i = 0;i < num;i++) {
			scanf ("%d", &arr[i]);
		}	
			
		int flag = RepeatNum (arr, num);
			
		if (flag == -1) {
			printf ("No repeating elements\n");
		}	
		else {
			printf ("Repeating elements: %d\n", flag);
		}	
	}		
	return 0;
}