5.最長的迴文子串

暴力解法

思路

1、做一個子函數，用於檢測輸入的字符串是否是迴文串
2、使用雙指針，頭指針從字符串開始處遍歷，尾指針每次均從結尾處開始，檢查頭尾指針之間的字符串是否是迴文串，若是，且長度大於之前的長度，則更新，否則進行下次檢查，注意，大循環的結束條件可以隨着找到迴文子串的長度而更新。

代碼

#include <stdio.h>
#include <string.h>
#include <stdlib.h>   // for malloc(), free()

#define YES 1
#define NO 	0

int IsPalindrome(char *strIn,int strLength)
{
	for (int i = 0; i < strLength/2; i++) {
		if (strIn[i] != strIn[strLength - 1 - i]) {
			return NO;
		}
	}
	return YES;
}

char * longestPalindrome(char * s)
{
	int strLength = strlen(s);
	if (strLength < 2) {
		return s;
	}

	char tmpLongestPald[1001] = {0};
    tmpLongestPald[0] = s[0];
	int maxLength = 1;
	int endIndex;
	for (int i = 0; i < strLength - maxLength; i++) {
		endIndex = strLength - 1;
		while (endIndex - i+1 > maxLength) {
			if (IsPalindrome(s+i,endIndex-i+1)==YES) {
				if (endIndex - i + 1>maxLength) {
					maxLength = endIndex - i + 1;
					memcpy(tmpLongestPald, s + i, maxLength);
				}
				break;
			} else {
				endIndex--;
			}	
		}	
	}
    char* strRet;
    strRet = (char*)malloc(maxLength+1);
    strcpy(strRet,tmpLongestPald);
	return strRet;
}

優化

思路

1、從字符串頭開始遍歷
2、找到最小子串後，依次向外擴展
3、最小子串有兩種形式 bb 或者 aba
4、根據找到的最大長度選擇是否提前結束循環

代碼

char * longestPalindrome(char * s)
{
	int strLength = strlen(s);
	if (strLength < 2) {
		return s;
	}

	char tmpLongestPald[1001] = {0};
    tmpLongestPald[0] = s[0];
	int maxLength = 1;
	int sideSize = 0;
	for (int i = 0; i < strLength - maxLength/2; i++) {
		sideSize = 0;
		while ((i-sideSize >= 0) && (i + sideSize + 1 < strLength))
		{
			if (s[i-sideSize] == s[i+sideSize+1]) {
				sideSize++;
			} else {
				break;
			}
		}
		if (maxLength < 2 * sideSize) {
			maxLength = 2 * sideSize;
			memcpy(tmpLongestPald, s+i-sideSize+1, maxLength);
		}
		sideSize = 1;
		while ((i-sideSize >= 0) && (i + sideSize  < strLength))
		{
			if (s[i-sideSize] == s[i+sideSize]) {
				sideSize++;
			} else {
				break;
			}
		}
		if (maxLength < 2 * sideSize - 1 ) {
			maxLength = 2 * sideSize - 1;
			memcpy(tmpLongestPald, s+i-sideSize+1, maxLength);
		}
	}

	char* strRet;
	strRet = (char*)malloc(maxLength+1);
	strcpy(strRet,tmpLongestPald);
	return strRet;
}