#807 · #1846 · Maximum Element After Decreasing and Rearranging(减小和重新排列数组后的最大元素)

You are given an array of positive integers arr. You may rearrange the elements and decrease any element (but never increase one). The final array must satisfy two rules: its first element is exactly 1, and the absolute difference between every pair of adjacent elements is at most 1. Return the maximum possible value of the last element. Constraints: 1 <= arr.length <= 10^5, 1 <= arr[i] <= 10^9.