[Leetcode] 340. Longest Substring with At Most K Distinct Characters 解题报告
2017-08-08 11:14
507 查看
题目:
Given a string, find the length of the longest substring T that contains at most k distinct characters.
For example, Given s =
T is "ece" which its length is 3.
思路:
一道two pointers的题目:我们维护一个哈希表,记录当前有效子字符串中出现的字符以及其对应个数,以及当前合法子字符串的起始位置start。在扫描字符串的过程中,如果发现当前字符在哈希表中已经出现了,那么直接更新哈希表中相应字符的出现次数即可;否则就需要将该字符添加到哈希表中。但是注意添加之后有可能会导致哈希表的容量大于k,这时就要移动起始位置start,并且更新哈希表,直到哈希表的容量重新不大于k。该算法的空间复杂度是O(k),因为哈希表中最多有k + 1个字符。时间复杂度是O(n)(请读者分析,for循环里面出现了while循环,为什么时间复杂度还是O(n)呢?答案:用后向分析可知,字符串中的每个字符最多加入哈希表一次,弹出哈希表一次,所以while循环的整个执行次数不超过2n次,其中n是字符串的长度。)
代码:
class Solution {
public:
int lengthOfLongestSubstringKDistinct(string s, int k) {
unordered_map<char, int> hash;
int start = 0, ans = 0;
for (int i = 0; i < s.length(); ++i) {
if (hash.count(s[i]) == 0) {
hash[s[i]] = 1;
while(hash.size() > k) {
--hash[s[start]];
if (hash[s[start]] == 0) {
hash.erase(s[start]);
}
++start;
}
}
else {
++hash[s[i]];
}
ans = max(ans, i - start + 1);
}
return ans;
}
};
Given a string, find the length of the longest substring T that contains at most k distinct characters.
For example, Given s =
“eceba”and k = 2,
T is "ece" which its length is 3.
思路:
一道two pointers的题目:我们维护一个哈希表,记录当前有效子字符串中出现的字符以及其对应个数,以及当前合法子字符串的起始位置start。在扫描字符串的过程中,如果发现当前字符在哈希表中已经出现了,那么直接更新哈希表中相应字符的出现次数即可;否则就需要将该字符添加到哈希表中。但是注意添加之后有可能会导致哈希表的容量大于k,这时就要移动起始位置start,并且更新哈希表,直到哈希表的容量重新不大于k。该算法的空间复杂度是O(k),因为哈希表中最多有k + 1个字符。时间复杂度是O(n)(请读者分析,for循环里面出现了while循环,为什么时间复杂度还是O(n)呢?答案:用后向分析可知,字符串中的每个字符最多加入哈希表一次,弹出哈希表一次,所以while循环的整个执行次数不超过2n次,其中n是字符串的长度。)
代码:
class Solution {
public:
int lengthOfLongestSubstringKDistinct(string s, int k) {
unordered_map<char, int> hash;
int start = 0, ans = 0;
for (int i = 0; i < s.length(); ++i) {
if (hash.count(s[i]) == 0) {
hash[s[i]] = 1;
while(hash.size() > k) {
--hash[s[start]];
if (hash[s[start]] == 0) {
hash.erase(s[start]);
}
++start;
}
}
else {
++hash[s[i]];
}
ans = max(ans, i - start + 1);
}
return ans;
}
};
相关文章推荐
- [leetcode] 340. Longest Substring with At Most K Distinct Characters 解题报告
- [leetcode] 159. Longest Substring with At Most Two Distinct Characters 解题报告
- [Leetcode] 159. Longest Substring with At Most Two Distinct Characters 解题报告
- LeetCode 340. Longest Substring with At Most K Distinct Characters
- LeetCode 340. Longest Substring with At Most K Distinct Characters(最长字串)
- Leetcode Everyday: 340. Longest Substring with At Most K Distinct Characters
- [LeetCode] 340. Longest Substring with At Most K Distinct Characters 最多有K个不同字符的最长子串
- leetcode 340. Longest Substring with At Most K Distinct Characters
- LeetCode 340. Longest Substring with At Most K Distinct Characters
- Leetcode-159.Longest Substring with At Most Two Distinct Characters
- Leetcode: Longest Substring with At Most Two Distinct Characters
- Leetcode: Longest Substring with At Most Two Distinct Characters
- [LeetCode] 159. Longest Substring with At Most Two Distinct Characters 最多有两个不同字符的最长子串
- LeetCode Longest Substring with At Most Two Distinct Characters
- Leetcode Longest Substring with At Most Two Distinct Characters
- LeetCode 159. Longest Substring with At Most Two Distinct Characters(最长字串)
- Leetcode 159 Longest Substring with At Most Two Distinct Characters
- [LeetCode] Longest Substring with At Most Two Distinct Characters及扩展
- 【LeetCode】Longest Substring with At Most Two Distinct Characters (2 solutions)
- [LeetCode]159. Longest Substring with At Most Two Distinct Characters