数据库索引与事务管理 B+ tree B+树是n叉排序树 hash index MySQL: innoDB以及MyISAM引擎，均不支持Hash索引

ES6 let则实际上为JavaScript新增了块级作用域。用它所声明的变量，只在let命令所在的代码块内有效。 1 2 3 4 5 6 7 var a = []; for (let i = 0;

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Statistical Learning Videos Book: Fundamentals of Modern Statistical Methods: Substantially Improving Power and Accuracy Supervised Learning Regression and classification is classified by its output types. Regression when we predict quantitative outputs, and clas- sification when we predict qualitative outputs. Least squares assumes f(x) is well approximated by a globally linear function. k-nearest neighbors assumes f(x) is well approximated by a locally constant function. One fact should be clear by now.

99. Recover Binary Search Tree travese in order. The result should be sorted ascending. The misplaced larger element will be close to head, while the smaller one will be close to end. For example, 1 2 3 is the in order. But the wrong seq is like 3 2 1. Apparently, 3 > 2 so 3 is the first node. 2 > 1, so 1 is the second one.