5. Longest Palindromic Substring
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Description:
Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.
Example:
Input: "babad"Output: "bab"Note: "aba" is also a valid answer.
Example:
Input: "cbbd"Output: "bb"
思路分析:首先应该可以想到的方法是通过逐一比较的方式去解决问题。遍历字符串当中的每一个元素,并以当前元素为中心,寻找最长的回文字符串。但是应当注意的问题是回文的形式是有两种的,一种是“aba”形式的,另一种则是“bb”形式的,一定要在开始的时候就进行判断。
class Solution {public: string longestPalindrome(string s) { int startIdx = 0, left = 0, right = 0, len = 0; for (int i = 0; i < s.size() - 1; ++i) { if (s[i] == s[i + 1]) { left = i; right = i + 1; searchPalindrome(s, left, right, startIdx, len); } left = right = i; searchPalindrome(s, left, right, startIdx, len); } if (len == 0) len = s.size(); return s.substr(startIdx, len); } void searchPalindrome(string s, int left, int right, int &startIdx, int &len) { int step = 1; while ((left - step) >= 0 && (right + step) < s.size()) { if (s[left - step] != s[right + step]) break; ++step; } int wide = right - left + 2 * step - 1; if (len < wide) { len = wide; startIdx = left - step + 1; } }};
第二种方法当时我是没有想到的,后来在网上看到别人有说可以通过动态规划的方法去实现,而且代码比较简单。然后我大致地看了一下,他是利用一个bool类型的二维数组去记录两两元素之间的关系。比如说它先判断了相邻的两个元素是否相同,并在二维数组当中记录;随后再次判断间隔距离为1的,然后步长逐步加大。感觉这种有点难以理解,不过为了方便学习也在这里贴出来。。
//动态规划算法 string longestPalindromeDP(string s) { int n = s.length(); int longestBegin = 0; int maxLen = 1; bool table[1000][1000] = {false}; for (int i = 0; i < n; i++) { table[i][i] = true; } for (int i = 0; i < n-1; i++) { if (s[i] == s[i+1]) { table[i][i+1] = true; longestBegin = i; maxLen = 2; } } for (int len = 3; len <= n; len++) { for (int i = 0; i < n-len+1; i++) { int j = i+len-1; if (s[i] == s[j] && table[i+1][j-1]) { table[i][j] = true; longestBegin = i; maxLen = len; } } } return s.substr(longestBegin, maxLen); }
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- 5.Longest Palindromic Substring
- 5. Longest Palindromic Substring
- 5. Longest Palindromic Substring
- 5.Longest Palindromic Substring
- 5. Longest Palindromic Substring
- 5.Longest Palindromic Substring
- 5. Longest Palindromic Substring
- 5. Longest Palindromic Substring
- 5. Longest Palindromic Substring
- 5.Longest Palindromic Substring
- 5. Longest Palindromic Substring
- 5. Longest Palindromic Substring
- 5. Longest Palindromic Substring
- 5. Longest Palindromic Substring
- 5. Longest Palindromic Substring
- 5. Longest Palindromic Substring
- 5. Longest Palindromic Substring
- 5. Longest Palindromic Substring
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