LintCode Singleton

Singleton 3 大要素：

1.有private static的句柄（成员变量即field）

2. constructor 必须为private

3.有public static的getInstance方法

class Solution {
    private static Solution instance;

    private Solution(){

    }
    /**
     * @return: The same instance of this class every time
     */
    public static Solution getInstance() {
        if(instance == null){
            instance = new Solution();
        }
        return instance;
    }
};