Find all polynomials $P$ with integer coefficients such that  $$s(x)=s(y) \implies s(|P(x)|)=s(|P(y)|).$$
 for all $x,y\in \mathbb{N}$.
Note: $s(x)$ denotes the sum of digits of $x$.

[i]Proposed by Şevket Onur YILMAZ[/i]
1. **Case 1: \( P(x) \) is a constant polynomial**

   If \( P(x) \) is a constant polynomial, say \( P(x) = c \), then \( |P(x)| = |c| \) for all \( x \). Since \( s(|c|) \) is a constant, the condition \( s(x) = s(y) \implies s(|P(x)|) = s(|P(y)|) \) holds trivially. 

2. **Case 2: \( P(x) \) is a non-constant polynomial**

   Assume \( P(x) \) is not a constant polynomial. Let \( P(x) = a_n x^n + a_{n-1} x^{n-1} + \cdots + a_1 x + a_0 \) where \( a_i \in \mathbb{Z} \) and \( a_n \neq 0 \).

3. **Non-negativity of coefficients**

   We first show that all coefficients \( a_i \) must be non-negative. Assume for contradiction that there exists \( k \) such that \( a_k < 0 \). Let \( k \) be the largest such index. Then for sufficiently large \( m \), \( P(10^m) \) will have a large negative term \( a_k 10^{km} \) which will dominate the polynomial, making \( P(10^m) \) negative. This contradicts the requirement that \( s(|P(10^m)|) = s(|P(1)|) \) for all \( m \geq 0 \), as the sum of digits of a negative number is not defined in the same way. Hence, all \( a_i \geq 0 \).

4. **Degree of the polynomial**

   Next, we show that the degree of \( P(x) \) must be 1. Assume \( \deg(P) > 1 \). Consider \( P(10^m x) \):
   \[
   P(10^m x) = a_n (10^m x)^n + a_{n-1} (10^m x)^{n-1} + \cdots + a_1 (10^m x) + a_0
   \]
   For large \( m \), the polynomial \( P(10^m x) \) will have terms with many zeros between significant digits, making the sum of digits of \( P(10^m x) \) very large. This contradicts the requirement that \( s(P(x)) = s(P(10^m x)) \). Therefore, \( \deg(P) = 1 \).

5. **Form of the polynomial**

   Let \( P(x) = ax + b \). We need to show that \( a = 10^k \) and \( 0 \leq b < 10^k \). Assume \( a \neq 10^k \). Consider \( x = c \cdot 10^k + d \cdot 10^m \) where \( c, d < 10 \) and \( k, m \) are chosen such that the significant digits of \( ac \cdot 10^k \) and \( ad \cdot 10^m \) overlap. By choosing \( c \) and \( d \) appropriately, we can ensure that the sum of digits of \( P(x) \) exceeds the sum of digits of \( P(y) \) for some \( y \), contradicting the given condition. Hence, \( a = 10^k \).

6. **Bound on \( b \)**

   If \( b \geq 10^k \), consider \( x = 9 \cdot 10^m \) for large \( m \). The sum of digits of \( P(x) = 10^k \cdot 9 \cdot 10^m + b \) will be large, contradicting the condition. Therefore, \( 0 \leq b < 10^k \).

Conclusion:
The only non-constant polynomials \( P(x) \) that satisfy the given condition are of the form \( P(x) = \pm (10^k x + c) \) where \( 0 \leq c < 10^k \).

The final answer is \( \boxed{ P(x) = \pm (10^k x + c) } \) where \( 0 \leq c < 10^k \).