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package leecode;
import java.util.*;
/**
* 5. 最长回文子串
*/
public class Solution5 {
public static void main(String[] args) {
Solution5 s = new Solution5();
System.out.println(s.longestPalindrome3("abcda"));
System.out.println(s.longestPalindrome3("babad"));
System.out.println(s.longestPalindrome3("aaaaa"));
System.out.println(s.longestPalindrome3("ababababa"));
System.out.println(s.longestPalindrome3("jjhg上海自来水来自海上oo"));
}
/**
* 使用双指针(收敛)
*/
public String longestPalindrome(String s) {
if (s == null || s.isEmpty()) {
return s;
}
char[] chars = s.toCharArray();
int length = chars.length;
if (length == 2) {
return chars[0] == chars[1] ? s : String.valueOf(chars[0]);
}
// 增加索引提速
Map<Character, List<Integer>> indexCache = new HashMap<>();
for (int i = 0; i < length; i++) {
indexCache.computeIfAbsent(chars[i], c->new ArrayList<>()).add(i);
}
int maxLength = 0;
int start = 0;
for (int i = 0; i + maxLength < length; i++) {
List<Integer> indexArr = indexCache.get(chars[i]);
// if (indexArr != null) {
// indexCache.put(chars[i], null);
// } else {
// continue;
// }
for (int j = indexArr.size() - 1; j >= 0; j--) {
int left = i;
int right = indexArr.get(j);
if (left + maxLength > right) {
break;
}
while (left <= right && right >= 0 && chars[left] == chars[right]) {
// success
if (left == right || left + 1 == right) {
int w = indexArr.get(j) - i;
if (w >= maxLength) {
maxLength = w;
start = i;
}
break;
} else {
left++;
right--;
}
}
}
}
return s.substring(start, start + maxLength + 1);
}
/**
* 使用双指针(扩散)
*/
public String longestPalindrome2(String s) {
if (s == null || s.isEmpty()) {
return s;
}
if (s.length() == 1) {
return s;
}
char[] chars = s.toCharArray();
int[] max = new int[2];
for (int i = 0; i < chars.length; i++) {
maxLength(chars, i, i, max);
maxLength(chars, i, i + 1, max);
}
return s.substring(max[0], max[1]+1);
}
void maxLength(char[]arr, int i, int right, int[]max) {
int left = i, /*right = i+1,*/ l = i, r = i;
while (left >=0 && right < arr.length && arr[left] == arr[right]) {
l = left--;
r = right++;
}
if ((r - l) > (max[1] - max[0])) {
max[0] = l;
max[1] = r;
}
}
/**
* 动态规划
*/
public String longestPalindrome3(String s) {
if (s == null || s.length() <=1) {
return s;
}
char[] chars = s.toCharArray();
int length = chars.length;
if (length == 2) {
return chars[0] == chars[1] ? s : String.valueOf(chars[0]);
}
// dp[i][j] = dp[i+1][j-1] && chars[i]==chars[j]
boolean[][] dp = new boolean[length][length];
// init
for (int i = 0; i < length; i++) dp[i][i] = true;
for (int i = 1; i < length; i++) dp[i-1][i] = chars[i-1] == chars[i];
// 依赖左下角结果
// 从下往上，从右往左遍历
for (int i = length-3; i >= 0; i--) {
for (int j = i+2; j < length; j++) {
dp[i][j] = dp[i+1][j-1] && chars[i]==chars[j];
}
}
// 从右上角开始朝左下角遍历
for (int j = length-1, m = j; j > 0; j--, m=j) {
for (int i = 0; m < length && i < length; i++, m++) {
if (dp[i][m]) {
return s.substring(i, m + 1);
}
}
}
return String.valueOf(chars[0]);
}
}