解析

class Solution {
public:
  int uniquePaths(int m, int n) {
    // long res = 1; long类型在win下是32位
    long long res =1;
    for (int x = n, y = 1; y < m; ++x, ++y) {
      // res *= x / y; 会导致先运算x/y导致错误
      res = res * x / y;
    }
    return res;
  }
};

class Solution {
public:
  int uniquePaths(int m, int n) {
    vector<vector<int>> DP(m, vector<int>(n));
    for (int i = 0; i < m; ++i) {
      DP[i][0] = 1;
    }
    for (int i = 0; i < n; ++i) {
      DP[0][i] = 1;
    }
    for (int i = 1; i < m; ++i) {
      for (int j = 1; j < n; ++j) {
        DP[i][j] = DP[i - 1][j] + DP[i][j - 1];
      }
    }
    return DP[m - 1][n - 1];
  }
};