Let $P_1,P_2,\dots,P_{720}$ denote the integers whose digits are a permutation of $123456$, arranged in ascending order (so $P_1=123456$, $P_2=123465$, and $P_{720}=654321$). What is $P_{144}$?
To find \( P_{144} \), we need to understand the permutations of the digits \( 1, 2, 3, 4, 5, 6 \) and how they are ordered.

1. **Calculate the total number of permutations:**
   The total number of permutations of 6 digits is given by \( 6! \):
   \[
   6! = 720
   \]
   This means there are 720 permutations in total.

2. **Determine the first digit of \( P_{144} \):**
   Each digit can be the first digit in \( 5! \) permutations of the remaining 5 digits:
   \[
   5! = 120
   \]
   Since \( 120 < 144 \leq 240 \), the first digit of \( P_{144} \) must be 2 (because the first 120 permutations start with 1, and the next 120 permutations start with 2).

3. **Determine the second digit of \( P_{144} \):**
   We now consider the permutations of the remaining digits \( \{1, 3, 4, 5, 6\} \) and need to find the 24th permutation within this group (since \( 144 - 120 = 24 \)):
   \[
   4! = 24
   \]
   Since \( 24 \leq 24 \), the second digit must be the last in the first group of permutations starting with 2. Therefore, the second digit is 1.

4. **Determine the third digit of \( P_{144} \):**
   We now consider the permutations of the remaining digits \( \{3, 4, 5, 6\} \):
   \[
   3! = 6
   \]
   Since \( 24 = 4 \times 6 \), the third digit must be the last in the fourth group of permutations starting with 21. Therefore, the third digit is 6.

5. **Determine the fourth digit of \( P_{144} \):**
   We now consider the permutations of the remaining digits \( \{3, 4, 5\} \):
   \[
   2! = 2
   \]
   Since \( 24 - 18 = 6 \) (where 18 is the number of permutations of the first three groups), the fourth digit must be the last in the third group of permutations starting with 216. Therefore, the fourth digit is 5.

6. **Determine the fifth digit of \( P_{144} \):**
   We now consider the permutations of the remaining digits \( \{3, 4\} \):
   \[
   1! = 1
   \]
   Since \( 6 - 4 = 2 \) (where 4 is the number of permutations of the first two groups), the fifth digit must be the last in the second group of permutations starting with 2165. Therefore, the fifth digit is 4.

7. **Determine the sixth digit of \( P_{144} \):**
   The remaining digit is 3.

Thus, the 144th permutation \( P_{144} \) is:
\[
P_{144} = 216543
\]

The final answer is \( \boxed{216543} \).