寻找中位数(分治法）

function medianOfMedians(list, left, right)
     numMedians = ceil((right - left) / 5) //每个子数组有5个元素，numMedians是子数组的数量 
     for i from 0 to numMedians //顺序处理每个子数组
         subLeft  := left + i*5
         subRight  := subLeft + 4
         if (subRight > right) subRight  := right
         // selectIdx是寻找子数组中位数的一个算法
         medianIdx  := selectIdx(list, subLeft, subRight, (subRight - subLeft) / 2)
         swap list[left+i] and list[medianIdx] //把子数组的中位数移到list的开头
     return selectIdx(list, left, left + numMedians - 1, numMedians / 2) //计算中位数的中位数