Among all leetcode questions, I find that there are at least 5 substring search problem which could be solved by the sliding window algorithm. so I sum up the algorithm template here. wish it will help you!
- *the template*:
public class Solution {
public List<Integer> slidingWindowTemplateByHarryChaoyangHe(String s, String t) {
//init a collection or int value to save the result according the question.
List<Integer> result = new LinkedList<>();
if(t.length()> s.length()) return result;
//create a hashmap to save the Characters of the target substring.
//(K, V) = (Character, Frequence of the Characters)
Map<Character, Integer> map = new HashMap<>();
for(char c : t.toCharArray()){
map.put(c, map.getOrDefault(c, 0) + 1);
}
//maintain a counter to check whether match the target string.
int counter = map.size();//must be the map size, NOT the string size because the char may be duplicate.
//Two Pointers: begin - left pointer of the window; end - right pointer of the window
int begin = 0, end = 0;
//the length of the substring which match the target string.
int len = Integer.MAX_VALUE;
//loop at the begining of the source string
while(end < s.length()){
char c = s.charAt(end);//get a character
if( map.containsKey(c) ){
map.put(c, map.get(c)-1);// plus or minus one
if(map.get(c) == 0) counter--;//modify the counter according the requirement(different condition).
}
end++;
//increase begin pointer to make it invalid/valid again
while(counter == 0 /* counter condition. different question may have different condition */){
char tempc = s.charAt(begin);//***be careful here: choose the char at begin pointer, NOT the end pointer
if(map.containsKey(tempc)){
map.put(tempc, map.get(tempc) + 1);//plus or minus one
if(map.get(tempc) > 0) counter++;//modify the counter according the requirement(different condition).
}
/* save / update(min/max) the result if find a target*/
// result collections or result int value
begin++;
}
}
return result;
}
}
- Firstly, here is my sliding solution this question. I will sum up the template below this code.
2) the similar questions are:
https://leetcode.com/problems/minimum-window-substring/ https://leetcode.com/problems/longest-substring-without-repeating-characters/ https://leetcode.com/problems/substring-with-concatenation-of-all-words/ https://leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/ https://leetcode.com/problems/find-all-anagrams-in-a-string/
3) I will give my solution for these questions use the above template one by one
Minimum-window-substring https://leetcode.com/problems/minimum-window-substring/
public class Solution {
public String minWindow(String s, String t) {
if(t.length()> s.length()) return "";
Map<Character, Integer> map = new HashMap<>();
for(char c : t.toCharArray()){
map.put(c, map.getOrDefault(c,0) + 1);
}
int counter = map.size();
int begin = 0, end = 0;
int head = 0;
int len = Integer.MAX_VALUE;
while(end < s.length()){
char c = s.charAt(end);
if( map.containsKey(c) ){
map.put(c, map.get(c)-1);
if(map.get(c) == 0) counter--;
}
end++;
while(counter == 0){
char tempc = s.charAt(begin);
if(map.containsKey(tempc)){
map.put(tempc, map.get(tempc) + 1);
if(map.get(tempc) > 0){
counter++;
}
}
if(end-begin < len){
len = end - begin;
head = begin;
}
begin++;
}
}
if(len == Integer.MAX_VALUE) return "";
return s.substring(head, head+len);
}
}
you may find that I only change a little code above to solve the question “Find All Anagrams in a String”: change
if(end-begin < len){
len = end - begin;
head = begin;
}
to
if(end-begin == t.length()){
result.add(begin);
}
longest substring without repeating characters https://leetcode.com/problems/longest-substring-without-repeating-characters/
public class Solution {
public int lengthOfLongestSubstring(String s) {
Map<Character, Integer> map = new HashMap<>();
int begin = 0, end = 0, counter = 0, d = 0;
while (end < s.length()) {
// > 0 means repeating character
//if(map[s.charAt(end++)]-- > 0) counter++;
char c = s.charAt(end);
map.put(c, map.getOrDefault(c, 0) + 1);
if(map.get(c) > 1) counter++;
end++;
while (counter > 0) {
//if (map[s.charAt(begin++)]-- > 1) counter--;
char charTemp = s.charAt(begin);
if (map.get(charTemp) > 1) counter--;
map.put(charTemp, map.get(charTemp)-1);
begin++;
}
d = Math.max(d, end - begin);
}
return d;
}
}
Longest Substring with At Most Two Distinct Characters https://leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/
public class Solution {
public int lengthOfLongestSubstringTwoDistinct(String s) {
Map<Character,Integer> map = new HashMap<>();
int start = 0, end = 0, counter = 0, len = 0;
while(end < s.length()){
char c = s.charAt(end);
map.put(c, map.getOrDefault(c, 0) + 1);
if(map.get(c) == 1) counter++;//new char
end++;
while(counter > 2){
char cTemp = s.charAt(start);
map.put(cTemp, map.get(cTemp) - 1);
if(map.get(cTemp) == 0){
counter--;
}
start++;
}
len = Math.max(len, end-start);
}
return len;
}
}
Substring with Concatenation of All Words https://leetcode.com/problems/substring-with-concatenation-of-all-words/
public class Solution {
public List<Integer> findSubstring(String S, String[] L) {
List<Integer> res = new LinkedList<>();
if (L.length == 0 || S.length() < L.length * L[0].length()) return res;
int N = S.length();
int M = L.length; // *** length
int wl = L[0].length();
Map<String, Integer> map = new HashMap<>(), curMap = new HashMap<>();
for (String s : L) {
if (map.containsKey(s)) map.put(s, map.get(s) + 1);
else map.put(s, 1);
}
String str = null, tmp = null;
for (int i = 0; i < wl; i++) {
int count = 0; // remark: reset count
int start = i;
for (int r = i; r + wl <= N; r += wl) {
str = S.substring(r, r + wl);
if (map.containsKey(str)) {
if (curMap.containsKey(str)) curMap.put(str, curMap.get(str) + 1);
else curMap.put(str, 1);
if (curMap.get(str) <= map.get(str)) count++;
while (curMap.get(str) > map.get(str)) {
tmp = S.substring(start, start + wl);
curMap.put(tmp, curMap.get(tmp) - 1);
start += wl;
//the same as https://leetcode.com/problems/longest-substring-without-repeating-characters/
if (curMap.get(tmp) < map.get(tmp)) count--;
}
if (count == M) {
res.add(start);
tmp = S.substring(start, start + wl);
curMap.put(tmp, curMap.get(tmp) - 1);
start += wl;
count--;
}
}else {
curMap.clear();
count = 0;
start = r + wl;//not contain, so move the start
}
}
curMap.clear();
}
return res;
}
}
Find All Anagrams in a String https://leetcode.com/problems/find-all-anagrams-in-a-string/
public class Solution {
public List<Integer> findAnagrams(String s, String t) {
List<Integer> result = new LinkedList<>();
if(t.length()> s.length()) return result;
Map<Character, Integer> map = new HashMap<>();
for(char c : t.toCharArray()){
map.put(c, map.getOrDefault(c, 0) + 1);
}
int counter = map.size();
int begin = 0, end = 0;
int head = 0;
int len = Integer.MAX_VALUE;
while(end < s.length()){
char c = s.charAt(end);
if( map.containsKey(c) ){
map.put(c, map.get(c)-1);
if(map.get(c) == 0) counter--;
}
end++;
while(counter == 0){
char tempc = s.charAt(begin);
if(map.containsKey(tempc)){
map.put(tempc, map.get(tempc) + 1);
if(map.get(tempc) > 0){
counter++;
}
}
if(end-begin == t.length()){
result.add(begin);
}
begin++;
}
}
return result;
}
}