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: ¤ ¤ ¤ ¤ ¤ ¤ ¤ ¤ Because the 4th row is incomplete, we return 3.
1+2+3...+k <= n 求k的最大值
C++(22ms):
1 class Solution { 2 public: 3 int arrangeCoins(int n) { 4 int left = 0 ; 5 int right = n ; 6 while(left <= right){ 7 int mid = left + (right - left) / 2 ; 8 if (0.5 * (1+mid) * mid <= n) 9 left = mid + 1; 10 else 11 right = mid - 1; 12 } 13 return left - 1 ; 14 } 15 };
java(61ms):
1 class Solution { 2 public int arrangeCoins(int n) { 3 return (int)((-1 + Math.sqrt(1 + 8 * (long)n)) / 2); 4 } 5 }