1234. Replace the Substring for Balanced String
You are given a string containing only 4 kinds of characters 'Q', 'W', 'E' and 'R'.
A string is said to be balanced if each of its characters appears n/4 times where n is the length of the string.
Return the minimum length of the substring that can be replaced with any other string of the same length to make the original string s balanced.
Return 0 if the string is already balanced.
Example 1:
Input: s = "QWER"
Output: 0
Explanation: s is already balanced.
Example 2:
Input: s = "QQWE"
Output: 1
Explanation: We need to replace a 'Q' to 'R', so that "RQWE" (or "QRWE") is balanced.
Example 3:
Input: s = "QQQW"
Output: 2
Explanation: We can replace the first "QQ" to "ER".
Example 4:
Input: s = "QQQQ"
Output: 3
Explanation: We can replace the last 3 'Q' to make s = "QWER".
Constraints:
1 <= s.length <= 10^5s.lengthis a multiple of4scontains only'Q','W','E'and'R'.
Hint：
- Use 2-pointers algorithm to make sure all amount of characters outside the 2 pointers are smaller or equal to n/4.
- That means you need to count the amount of each letter and make sure the amount is enough.
題目：有一個只含有 'Q', 'W', 'E', 'R' 四種字符，且長度爲 n 的字符串。假如在該字符串中，這四個字符都恰好出現 n/4 次，那麼它就是一個「平衡字符串」。給你一個這樣的字符串 s，請通過「替換一個子串」的方式，使原字符串 s 變成一個「平衡字符串」。你可以用和「待替換子串」長度相同的 任何 其他字符串來完成替換。請返回待替換子串的最小可能長度。
思路：雙指針，參考lee215。兩個指針間是待替換的子串，那麼我們只需要保證在兩指針區域外的字符的頻率都小於n/4。如對於QWERQQQQ，count[Q]=5，初始指針i=0，那麼指針j需走到倒數第三個位置，才能使得Q的字符數滿足條件，此時替換的子串爲QWERQQ，嘗試右移左指針i，此時count[Q]=3，調整右指針j，count[Q]=2，更新左指針…
class Solution {
public:
int balancedString(string s) {
unordered_map<int, int> count;
for (auto c : s)
count[c]++;
int n = s.size();
int k = n / 4;
int res = n;
for (int i = 0, j = 0; j < n; ++j) {
count[s[j]]--;
while (i < n && count['Q'] <= k && count['W'] <= k &&
count['E'] <= k && count['R'] <= k) {
res = min(res, j - i + 1);
count[s[i++]] += 1;
}
}
return res;
}
};
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