249 Group Shifted Strings
1. Question
Given a string, we can "shift" each of its letter to its successive letter, for example:"abc" -> "bcd"
. We can keep "shifting" which forms the sequence:
"abc" -> "bcd" -> ... -> "xyz"
Given a list of strings which contains only lowercase alphabets, group all strings that belong to the same shifting sequence.
For example, given:["abc", "bcd", "acef", "xyz", "az", "ba", "a", "z"]
,
A solution is:
[
["abc","bcd","xyz"],
["az","ba"],
["acef"],
["a","z"]
]
2. Implementation
(1) HashMap
思路: 这里比较巧妙的做法是根据每个词的character和它之前的character的偏移量作为key. 注意由于a - z = -25, 为了让计算的结果统一为正数,我们需要对结果 + 26 再 模26
class Solution {
public List<List<String>> groupStrings(String[] strings) {
List<List<String>> res = new ArrayList<>();
if (strings == null || strings.length == 0) {
return res;
}
Map<String, List<String>> map = new HashMap<>();
for (String str : strings) {
StringBuilder key = new StringBuilder();
for (int i = 1; i < str.length(); i++) {
int offset = (str.charAt(i) - str.charAt(i - 1) + 26) % 26;
key.append(offset);
}
String k = key.toString();
if (!map.containsKey(k)) {
map.put(k, new LinkedList<>());
}
map.get(k).add(str);
}
res = new ArrayList<>(map.values());
return res;
}
}
3. Time & Space Complexity
HashMap: 时间复杂度O(nL), 空间复杂度O(n)
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