A discrete memory less source has an alphabet of seven

A discrete memory less source has an alphabet of seven symbols whose probabilities of occurrence are as described here:
Symbol s0 s1s2s3s4s5s6
Probability 0.25 0.25 0.125 0.125 0.125 0.0625 0.0625
Compute the Huffman code for this source, moving a ?ocombined?? symbol as high as possible. Explain why the computed source code has an efficiency of 100 percent.