做題筆記：Longest Common Prefix (Java)
還滿難的一道題。
題目描述：
編寫一個函數來查找字符串數組中最長的共有前綴字符串。
如果沒有共有前綴，則返回一個空字符串""。
Example 1:
Input: strs = ["flower","flow","flight"]
Output: "fl"Example 2:
Input: strs = ["dog","racecar","car"]
Output: ""
Explanation: There is no common prefix among the input strings.Horizontal scanning:
自己嘗試的方法：
class Solution {
public String longestCommonPrefix(String[] strs)
{
int size = strs.length;
if (size == 0)
return "";
if (size == 1)
return strs[0];
Arrays.sort(strs);
int end = Math.min(strs[0].length(), strs[size-1].length());
int i = 0;
while (i < end && strs[0].charAt(i) == strs[size-1].charAt(i) )
i++;
String pre = strs[0].substring(0, i);
return pre;
}
}1 ms
Vertical scanning:
Solution 中提供的方法二：
public String longestCommonPrefix(String[] strs) {
if (strs == null || strs.length == 0) return "";
for (int i = 0; i < strs[0].length() ; i++){
char c = strs[0].charAt(i);
for (int j = 1; j < strs.length; j ++) {
if (i == strs[j].length() || strs[j].charAt(i) != c)
return strs[0].substring(0, i);
}
}
return strs[0];
}Divide and conquer:
Solution 中提供的方法三：
public String longestCommonPrefix(String[] strs) {
if (strs == null || strs.length == 0) return "";
return longestCommonPrefix(strs, 0 , strs.length - 1);
}
private String longestCommonPrefix(String[] strs, int l, int r) {
if (l == r) {
return strs[l];
}
else {
int mid = (l + r)/2;
String lcpLeft = longestCommonPrefix(strs, l , mid);
String lcpRight = longestCommonPrefix(strs, mid + 1,r);
return commonPrefix(lcpLeft, lcpRight);
}
}
String commonPrefix(String left,String right) {
int min = Math.min(left.length(), right.length());
for (int i = 0; i < min; i++) {
if ( left.charAt(i) != right.charAt(i) )
return left.substring(0, i);
}
return left.substring(0, min);
}Binary search:
Solution 中提供的方法四：
public String longestCommonPrefix(String[] strs) {
if (strs == null || strs.length == 0)
return "";
int minLen = Integer.MAX_VALUE;
for (String str : strs)
minLen = Math.min(minLen, str.length());
int low = 1;
int high = minLen;
while (low <= high) {
int middle = (low + high) / 2;
if (isCommonPrefix(strs, middle))
low = middle + 1;
else
high = middle - 1;
}
return strs[0].substring(0, (low + high) / 2);
}
private boolean isCommonPrefix(String[] strs, int len){
String str1 = strs[0].substring(0,len);
for (int i = 1; i < strs.length; i++)
if (!strs[i].startsWith(str1))
return false;
return true;
}