避免内存重叠memmove（）性能

#include <iostream>
#include <string.h>
using namespace std;
void* memmove(void *dst, const void *src, size_t count){
	// 容错处理
	if (dst == NULL || src == NULL){
		return NULL;
	}
	unsigned char *pdst = (unsigned char *)dst;
	const unsigned char *psrc = (const unsigned char *)src;
	//推断内存是否重叠
	bool flag1 = (pdst >= psrc && pdst < psrc + count);
	bool flag2 = (psrc >= pdst && psrc < pdst + count);
	if (flag1 || flag2){
		// 倒序拷贝
		while (count){
			*(pdst + count - 1) = *(psrc + count - 1);
			count--;
		}//while
	}
	else{
		// 拷贝
		while (count--){
			*pdst = *psrc;
			pdst++;
			psrc++;
		}//while
	}
	return dst;
}