Added PA_Chapter6 English Version
Translated the sub-chapter __Tree-Shaped Problems: All About the Balance__
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倒过来翻译了Summary和Exercises，因为有一部分翻译在公司里没有上传
Merge branch 'master' of https://github.com/owlman/PythonAlgorithms into PA_Chapter6_by_scaret
附录1的英语原文
附录1 30%
Added APPENDIX A & 1/2 B
Merge branch 'PA_Chapter6_by_scaret'
Merged branch PA_Chapter6 & PA_APPENDIX