最大流 push-relabel 算法相关的一个习题

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算法导论习题 26.5-5：Suppose that at some point in the execution of a push-relabel algorithm, there exists an integer 0<k<=|V|-1 for which no vertex has v.h=k. Show that all vertices with v.h>k are on the source side of a minimum cut.
想了很久也没有证明出来。