test.go

package main

import (
	"fmt"
	"sort"
)

type Triangle struct {
	h int
	w int
}

type Triangles []Triangle

func (t *Triangle) Area() (a int) {
	a = (t.h * t.w) / 2
	return
}

func (t Triangles) Less(i, j int) bool {
	if t[i].Area() < t[j].Area() {
		return true
	}
	return false
}

func (g Triangles) Swap(i, j int) {
	g[i], g[j] = g[j], g[i]
}

func (t Triangles) Len() int {
	return len(t)
}

func main() {

	tg := Triangles{
		Triangle{2, 3},
		Triangle{2, 8},
		Triangle{3, 9},
		Triangle{3, 8},
		Triangle{3, 18},
		Triangle{9, 1},
		Triangle{3, 8},
	}

	sort.Sort(tg)
	for _, t := range tg {
		fmt.Println(t)
	}

}