Today I have done leetcode's September LeetCoding Challenge with cpp.

September LeetCoding Challenge 3

Description

Unique Paths

There is a robot on an m x n grid. The robot is initially located at the top-left corner (i.e., grid[0][0]). The robot tries to move to the bottom-right corner (i.e., grid[m - 1][n - 1]). The robot can only move either down or right at any point in time.

Given the two integers m and n, return the number of possible unique paths that the robot can take to reach the bottom-right corner.

The test cases are generated so that the answer will be less than or equal to 2 * 109.

Example 1:

Input: m = 3, n = 7
Output: 28

Example 2:

Input: m = 3, n = 2
Output: 3
Explanation: From the top-left corner, there are a total of 3 ways to reach the bottom-right corner:
1. Right -> Down -> Down
2. Down -> Down -> Right
3. Down -> Right -> Down

Constraints:

• 1 <= m, n <= 100

Solution

constexpr int combination(int total, int pick) {
  long long result = 1;
  for(int i = 0; i < pick; ++i) {
    result *= (total - i);
    result /= (i + 1);
  }
  return result;
}

class Solution {
public:
  int uniquePaths(int m, int n) {
    return combination(m + n - 2, min(n - 1, m - 1));
  }
};

// Accepted
// 63/63 cases passed (0 ms)
// Your runtime beats 100 % of cpp submissions
// Your memory usage beats 98.48 % of cpp submissions (5.8 MB)