数组遍历

1. for ( item in arr){…}

2. for ( i in arr.indeces ) {…}
(遍历下标)

3. for ((index, item) in arr.withInfex()) {…} (遍历下标和元素)

4. arr.forEach {} ( 遍历元素 )

5. arr.forEachIndexed{index, item -> …}