Leetcode: Arranging Coins

You have a total of n coins that you want to form in a staircase shape, where every k-th row must have exactly k coins.

Given n, find the total number of full staircase rows that can be formed.

n is a non-negative integer and fits within the range of a 32-bit signed integer.

Example 1:

n = 5

The coins can form the following rows:
¤
¤ ¤
¤ ¤

Because the 3rd row is incomplete, we return 2.
Example 2:

n = 8

The coins can form the following rows:
¤
¤ ¤
¤ ¤ ¤
¤ ¤

Because the 4th row is incomplete, we return 3.

 1 public class Solution {
 2     public int arrangeCoins(int n) {
 3         long sum = 0;
 4         int count = 0;
 5         while (true) {
 6             sum = sum + count;
 7             if (n < sum) break;
 8             count++;
 9         }
10         return count - 1;
11     }
12 }

 1 public class Solution {
 2     public int arrangeCoins(int n) {
 3         int l=1, r=n;
 4         while (l <= r) {
 5             int m = l + (r-l)/2;
 6             if (0.5*m+0.5*m*m > n) {
 7                 r = m - 1; 
 8             }
 9             else l = m + 1;
10         }
11         return r;
12     }
13 }