深度优先

• 问题描述：这是一道似曾相识的小学奥数题_._._ + _._._ = _._._将1～9数字填入到等式并使等式成立
• 样例：173+286=459

问题转化：借助N数全排的思想，这个问题和全排问题一直，只需要找出1～9在这9个位置的全排，并判断等式的有效性即可。

func DeepFirstSearch(step int) {
        if step == n {
                if ((locations[0]+locations[3])*100 + (locations[1]+locations[4])*10 + locations[2] + locations[5]) == (locations[6]*100 + locations[7]*10 + locations[8]) {
                        fmt.Println(locations[0:3], " + ", locations[3:6], " = ", locations[6:9])
                }
                return
        }

        for i := 0; i < n; i++ {
                if 0 == marks[i] {
                        locations[step] = i
                        marks[i] = 1
                        DeepFirstSearch2(step + 1)
                        marks[i] = 0
                }
        }
        return
}