组合部分1.n个正整数写成一行,艾丽丝选两个相邻的数x、y(x>y,且x在y的左边).她用数对(y+1,x)或(x-1,x)来代替(x,y).证明:艾丽丝只能进行有限次上述操作.2.已知n是正整数.求集合{1,2,…,n}.中不交的元素对的数目的最大值,使得对于任意两个不同的元素对(a,b)和(c,d),均有 Combination part 1.n positive integers written in a line, Alice selects two adjacent numbers x, y (x> y, and x in the left of y). She used the number of pairs (y +1, x) or (x- 1, x) instead of (x, y). Proof: Alice can only do this operation for a limited number of times. 2. Known n is a positive integer. Find the set {1,2, ..., n} The maximum of the number such that for any two different pairs of elements (a, b) and (c, d), there are