LeetCode——Palindromic Substrings

Question

Given a string, your task is to count how many palindromic substrings in this string.

The substrings with different start indexes or end indexes are counted as different substrings even they consist of same characters.

Example 1:

Input: "abc"

Output: 3

Explanation: Three palindromic strings: "a", "b", "c".

Example 2:

Input: "aaa"

Output: 6

Explanation: Six palindromic strings: "a", "a", "a", "aa", "aa", "aaa".

Note:
The input string length won't exceed 1000.

Solution

动态规划，依次求解长度为1，2，3...的子字符串是否为回文，用一个二维数组纪录，同时用一个计数器统计回文子字符串的个数。

Code

class Solution {
public:
    int countSubstrings(string s) {
        if (s.length() == 0)
            return 0;
        int len = s.length();
        table.resize(len);
        for (int i = 0; i < len; i++)
            table[i].resize(len);

        int count = 0;
        for (int i = 0; i < len; i++) {
            table[i][i] = true;
            count++;
        }
        //  遍历长度为2，3...的子字符串是否为回文
        for (int i = 2; i <= len; i++) {
            for (int j = 0; j <= len - i; j++) {
                int start = j;
                int end = j + i - 1;
                if (s[start] == s[end]) {
                    if (start + 1 < end - 1) {
                        if (table[start + 1][end - 1] == true) {
                            table[start][end] = true;
                            count++;
                        } else {
                            table[start][end] = false;
                        }
                    } else {
                        table[start][end] = true;
                        count++;
                    }
                } else {
                    table[start][end] = false;
                }
            }
        }
        
        return count;
    }
    vector<vector<bool> >  table;
};