这不是上周周赛题目吗，计数就行了，难在如何判断序列是否合法。我当时的解法是两边扫描，第一次计数，第二次判断序列合法性。
```java
class Solution {
public int minNumberOfFrogs(String croakOfFrogs) {
if (croakOfFrogs.length() == 0)
return -1;
int count = 0, max = 0;
for (int i = 0; i < croakOfFrogs.length(); i++) {
if (croakOfFrogs.charAt(i) == 'c')
count++;
else if (croakOfFrogs.charAt(i) == 'k')
count--;
max = Math.max(max, count);
}
int lastC = 0, lastR = 0, lastO = 0, lastA = 0, lastK = 0;
while (lastK < croakOfFrogs.length() - 1) {
int idxC = croakOfFrogs.indexOf("c", lastC), idxR = croakOfFrogs.indexOf("r", lastR), idxO = croakOfFrogs.indexOf("o", lastO), idxA = croakOfFrogs.indexOf("a", lastA), idxK = croakOfFrogs.indexOf("k", lastK);
if (idxC == -1 | idxR == -1 || idxO == -1 || idxA == -1 || idxK == -1)
return -1;
if (idxC < idxR && idxR < idxO && idxO < idxA && idxA < idxK) {
} else
return -1;
lastC = idxC + 1;
lastR = idxR + 1;
lastO = idxO + 1;
lastA = idxA + 1;
lastK = idxK + 1;
}
return max;
}
}
```