主要是剪枝：分为总体和枝节
总体：sum%4！=0，max<sum/4
枝节：在于dfs的参数设置和条件判断，动态的判断是否进行枝节：在于df
阅读背景：
经典搜索 剪枝 hdu 1518 Given a set of sticks of various lengths, is it possible to join them end-to-end to form a square?
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