用golang写了例2的benchmark，的确像老师说的那样，ij方式要快! 速度比 102:9

Running tool: /usr/local/go/bin/go test -benchmem -run=^$ -bench ^(BenchmarkFor_IJ|BenchmarkFor_JI)$ -v

goos: darwin
goarch: amd64
BenchmarkFor_IJ-4      102     12360520 ns/op     1254901 B/op     0 allocs/op
BenchmarkFor_JI-4      9     113365872 ns/op    14222222 B/op     0 allocs/op
``` golang
package main

import "testing"

const n = 4000

func BenchmarkFor_IJ(b *testing.B) {
    var arr [n][n]int
    for c := 0; c < b.N; c++ {
        for i := 0; i < n; i++ {
            for j := 0; j < n; j++ {
                arr[i][j] = i + j
            }
        }
    }
}

func BenchmarkFor_JI(b *testing.B) {
    var arr [n][n]int
    for c := 0; c < b.N; c++ {
        for i := 0; i < n; i++ {
            for j := 0; j < n; j++ {
                arr[j][i] = i + j
            }
        }
    }
}
```