# Java 1ms solution with comments in the code

• Key points:

1. Generating the numbers in ascending order so it can be teminated when the number is greater than high.

2. Only need to actually generate the numbers having the same length of low and high for range checking. The total nubmers of other lengths can be counted with a dp function.

3. Using char arrays and placing char on both ends is faster than using adding String operation. In addtion, comparing char arrays directly should be faster than calling String.compareTo() function.

`````` public class Solution {
char[] singles = new char[]{'0','1','8'};
// Sorted by the first char in ascending order
char[][] pairs = new char[][]{ {'0','0'},{'1','1'},{'6','9'},{'8','8'},{'9','6'} };
char[] ans; // buffer of storing the number
int count = 0; // total count
char[] l; // char array of low
char[] h; // char array of high

// Compare two numbers' char array. Longer one is the larger one.
// If have same length then compare each char from left to right
// return positive when s2 > s1, 0 when s2==s1, nagetive when s2 < s1
int comp(char[] s1, char[] s2) {
int len1 = s1.length;
int len2 = s2.length;
if (len1 != len2) return len2-len1;
else {
for (int i=0; i < len1; i++) {
if (s1[i] != s2[i]) return s2[i]-s1[i];
}
return 0;
}
}
// Recursion for generating Strobogrammatic numbers of length n
// starting from both ends. As a result, the numbers are
// generate in ascending order.
// Therefore, when when a number is greater than high it returns false
// and then terminate the loop.
boolean helper(int n, int s, int e, int len) {
if (n==0) return false;
if (len==n) { // a resulting number
// checking the range
if ( comp(l, ans) >=0  && comp(ans, h) >=0 ) count++;
if ( comp(ans, h) < 0) return false; // the nubmer is greater than high
return true;
} else if (s==e) { // odd length at the middle position, apply single digit
for (int i =0 ; i< singles.length; i++) {
if ( ! ( s == 0 && i == 0 && n > 1)  ) { // first digit can't be 0
ans[s] = singles[i];
if ( ! helper( n, s+1, e-1, len+1) ) return false;
}
}
} else { // placing two digits at both ends
for (int i =0 ; i< pairs.length; i++) {
char[] pair = pairs[i];
if ( ! ( s == 0 && i == 0)  ) { // first digit can't be 0
ans[s] = pair[0];
ans[e] = pair[1];
if ( ! helper( n, s+1, e-1, len+2 ) ) return false;
}
}
}
return true;
}
// counting the total Strobogrammatic numbers of lengh n
// without considering the range
int counts(int n, int next) {
if (next<=0) return 0;
if (next==1) return 3;
if (next==2) return n==next? 4 :5; // first digit can't be 0
return n==next? 4 * counts(n, next-2) : 5 * counts(n, next-2);
}

public int strobogrammaticInRange(String low, String high) {
int low_len = low.length();
int high_len = high.length();
l = low.toCharArray();
h = high.toCharArray();
for (int i=low_len; i <= high_len; i++) {
// generating the numbers only when the length is equal to low or high
if (i== low_len || i== high_len) {
ans = new char[i];
helper(i, 0, i-1, 0);
} else {
// counting the total numbers without acctualy generating them
count+=counts(i,i);
}
}
return count;
}
``````

}

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