441. Arranging Coins

Posted 2020-12-14 wentiliangkaihua

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) {
        int res = 0;
        int le = n;
        for(int i = 1; i <= n; i++){
            if(le < i) return i-1;
            else{
                le -= i;
            }
        }
        return n;
    }
}

或者二分法

class Solution {
  public int arrangeCoins(int n) {
    long left = 0, right = n;
    long k, curr;
    while (left <= right) {
      k = left + (right - left) / 2;
      System.out.println(k);
      curr = k * (k + 1) / 2;
        
      if (curr == n) return (int)k;

      if (n < curr) {
        right = k - 1;
      } else {
        left = k + 1;
      }
    }
    return (int)right;
  }
}

Find the maximum kk such that k (k + 1)/2≤N.