LeetCode习题笔记——Longest Palindromic Substring
2017-10-28 21:35
405 查看
这次的问题是判断一个字符串里最长的“回文串”,即一个串从左边到右边和右边到左边是一样(如"aba")。下面是问题的具体描述:
Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is
1000.
Example:
Example:
其实这个问题可以用到刚刚学过的动态规划(dynamic programming),首先一个字母一定是回文的,之后再判断两边是否是相同,就可以判断之后的是否回文,以此类推找出最长的。这就是动态规划——把多阶段过程转化为一系列单阶段问题,利用各阶段之间的关系,逐个求解。下面给出解法:
class Solution {
public:
string longestPalindrome(string s) {
int n = s.length();
int lb = 0;
int max = 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;
lb = i;
max = 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;
lb = i;
max = len;
}
}
}
return s.substr(lb,max);
}
};
主要思路就是用一个二维数组记录这个串从下标i到下标j是否是一个回文,然后再判断两边,直到循环结束即可。代码十分简单,而要理解这个动态规划的思路以及如何使用,这是一个非常典型的例子。
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"
其实这个问题可以用到刚刚学过的动态规划(dynamic programming),首先一个字母一定是回文的,之后再判断两边是否是相同,就可以判断之后的是否回文,以此类推找出最长的。这就是动态规划——把多阶段过程转化为一系列单阶段问题,利用各阶段之间的关系,逐个求解。下面给出解法:
class Solution {
public:
string longestPalindrome(string s) {
int n = s.length();
int lb = 0;
int max = 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;
lb = i;
max = 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;
lb = i;
max = len;
}
}
}
return s.substr(lb,max);
}
};
主要思路就是用一个二维数组记录这个串从下标i到下标j是否是一个回文,然后再判断两边,直到循环结束即可。代码十分简单,而要理解这个动态规划的思路以及如何使用,这是一个非常典型的例子。
相关文章推荐
- (python)leetcode刷题笔记05 Longest Palindromic Substring
- 自己刷Leetcode的笔记:Longest Palindromic Substring
- 【LeetCode笔记】Longest Palindromic Substring
- LeetCode习题笔记——Longest Substring Without Repeating Characters
- [LeetCode]-005-Longest Palindromic Substring
- 【LeetCode】5. Longest Palindromic Substring
- [leetcode-Q5] Longest Palindromic Substring
- 【Leetcode】之Longest Palindromic Substring
- leetcode 5. Longest Palindromic Substring
- LeetCode题解:Longest Palindromic Substring
- Longest Palindromic Substring leetcode 5
- [LeetCode][JavaScript]Longest Palindromic Substring
- Longest Palindromic Substring--leetcode
- LeetCode之 Longest Palindromic Substring
- [LeetCode]--5. Longest Palindromic Substring
- leetcode 5 Longest Palindromic Substring
- leetcode 5. Longest Palindromic Substring
- LeetCode 5 Longest Palindromic Substring
- LeetCode Longest Palindromic Substring 最长回文子字符串 两种方法分析解答
- LeetCode 5 Longest Palindromic Substring