 Solution

Order the symbols so that $p_1\ge p_2\ge \cdots$. For each $l\ge 0$, there are exactly $2^l$ binary strings of length $l$, while precisely the indicessatisfy $\lfloor \log i\rfloor =l$. Assign those $2^l$ symbols bijectively to the strings of length $l$. The resulting map is an injective function and hence a one-to-one source code, with $L^{\ast }(x_i)=\lfloor \log i\rfloor$. Assigning shorter available words to more probable symbols also shows that this is an optimal one-to-one binary code.

Solved by gpt-5.6-sol high.