441. 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:
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¤ ¤
¤ ¤

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

Example 2:

n = 8

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

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

Rust Solution

struct Solution;

impl Solution {
    fn arrange_coins(n: i32) -> i32 {
        (((2 * n as i64) as f64 + 0.25).sqrt() - 0.5).floor() as i32
    }
}

#[test]
fn test() {
    assert_eq!(Solution::arrange_coins(5), 2);
    assert_eq!(Solution::arrange_coins(8), 3);
    assert_eq!(Solution::arrange_coins(1_804_289_383), 60070);
}

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