《中英双解》leetCode Arranging Coins (排列硬币)

Arranging Coins

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You have n coins and you want to build a staircase with these coins. The staircase consists of k rows where the ith row has exactly i coins. The last row of the staircase may be incomplete.

Given the integer n, return the number of complete rows of the staircase you will build.

你总共有 n 枚硬币，并计划将它们按阶梯状排列。对于一个由 k 行组成的阶梯，其第 i 行必须正好有 i 枚硬币。阶梯的最后一行 可能 是不完整的。

给你一个数字 n ，计算并返回可形成 完整阶梯行 的总行数。

Example 1:

Input: n = 5
Output: 2
Explanation: Because the 3rd row is incomplete, we return 2.

Example 2:

Input: n = 8
Output: 3
Explanation: Because the 4th row is incomplete, we return 3.

class Solution {
    public int arrangeCoins(int n) {
       long left = 0;
       long right = n;
       long k,curr;
       while(left <= right){
           k = left + (right - left) / 2;
           //计算层数所有的硬币值
           curr = k * (k + 1) / 2;
           if(n == curr) return (int)k;
           if(n < curr){
               right = k - 1;
           } else {
               left = k + 1;
           }
       }
       return (int)right;
       
    }
}