Leetcode 441. Arranging Coins (Easy) (cpp)

Leetcode 441. Arranging Coins (Easy) (cpp)

Tag: Binary Search, Math

Difficulty: Easy

/*

441. Arranging Coins (Easy)

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.

*/
class Solution {
public:
	int arrangeCoins(int n) {
		if (n == 0) {
			return 0;
		}
		int l = 1, r = n;
		while (l < r) {
			long mid = l + (r - l + 1) / 2;
			if (mid * (mid + 1) / 2.0 <= n) {
				l = mid;
			}
			else if (mid * (mid + 1) / 2 > n) {
				r = mid - 1;
			}
		}
		return r;
	}
};