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62. Unique Paths

Description

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

Solutions

Solution 1: Dynamic Programming

We define \(f[i][j]\) to represent the number of paths from the top left corner to \((i, j)\), initially \(f[0][0] = 1\), and the answer is \(f[m - 1][n - 1]\).

Consider \(f[i][j]\):

  • If \(i > 0\), then \(f[i][j]\) can be reached by taking one step from \(f[i - 1][j]\), so \(f[i][j] = f[i][j] + f[i - 1][j]\);
  • If \(j > 0\), then \(f[i][j]\) can be reached by taking one step from \(f[i][j - 1]\), so \(f[i][j] = f[i][j] + f[i][j - 1]\).

Therefore, we have the following state transition equation:

\[ f[i][j] = \begin{cases} 1 & i = 0, j = 0 \\ f[i - 1][j] + f[i][j - 1] & \textit{otherwise} \end{cases} \]

The final answer is \(f[m - 1][n - 1]\).

The time complexity is \(O(m \times n)\), and the space complexity is \(O(m \times n)\). Here, \(m\) and \(n\) are the number of rows and columns of the grid, respectively.

We notice that \(f[i][j]\) is only related to \(f[i - 1][j]\) and \(f[i][j - 1]\), so we can optimize the first dimension space and only keep the second dimension space, resulting in a time complexity of \(O(m \times n)\) and a space complexity of \(O(n)\).

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class Solution:
    def uniquePaths(self, m: int, n: int) -> int:
        f = [[0] * n for _ in range(m)]
        f[0][0] = 1
        for i in range(m):
            for j in range(n):
                if i:
                    f[i][j] += f[i - 1][j]
                if j:
                    f[i][j] += f[i][j - 1]
        return f[-1][-1]

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class Solution {
    public int uniquePaths(int m, int n) {
        var f = new int[m][n];
        f[0][0] = 1;
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (i > 0) {
                    f[i][j] += f[i - 1][j];
                }
                if (j > 0) {
                    f[i][j] += f[i][j - 1];
                }
            }
        }
        return f[m - 1][n - 1];
    }
}

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

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func uniquePaths(m int, n int) int {
    f := make([][]int, m)
    for i := range f {
        f[i] = make([]int, n)
    }
    f[0][0] = 1
    for i := 0; i < m; i++ {
        for j := 0; j < n; j++ {
            if i > 0 {
                f[i][j] += f[i-1][j]
            }
            if j > 0 {
                f[i][j] += f[i][j-1]
            }
        }
    }
    return f[m-1][n-1]
}

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function uniquePaths(m: number, n: number): number {
    const f: number[][] = Array(m)
        .fill(0)
        .map(() => Array(n).fill(0));
    f[0][0] = 1;
    for (let i = 0; i < m; ++i) {
        for (let j = 0; j < n; ++j) {
            if (i > 0) {
                f[i][j] += f[i - 1][j];
            }
            if (j > 0) {
                f[i][j] += f[i][j - 1];
            }
        }
    }
    return f[m - 1][n - 1];
}

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impl Solution {
    pub fn unique_paths(m: i32, n: i32) -> i32 {
        let (m, n) = (m as usize, n as usize);
        let mut f = vec![1; n];
        for i in 1..m {
            for j in 1..n {
                f[j] += f[j - 1];
            }
        }
        f[n - 1]
    }
}

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/**
 * @param {number} m
 * @param {number} n
 * @return {number}
 */
var uniquePaths = function (m, n) {
    const f = Array(m)
        .fill(0)
        .map(() => Array(n).fill(0));
    f[0][0] = 1;
    for (let i = 0; i < m; ++i) {
        for (let j = 0; j < n; ++j) {
            if (i > 0) {
                f[i][j] += f[i - 1][j];
            }
            if (j > 0) {
                f[i][j] += f[i][j - 1];
            }
        }
    }
    return f[m - 1][n - 1];
};

Solution 2

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class Solution:
    def uniquePaths(self, m: int, n: int) -> int:
        f = [[1] * n for _ in range(m)]
        for i in range(1, m):
            for j in range(1, n):
                f[i][j] = f[i - 1][j] + f[i][j - 1]
        return f[-1][-1]

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class Solution {
    public int uniquePaths(int m, int n) {
        var f = new int[m][n];
        for (var g : f) {
            Arrays.fill(g, 1);
        }
        for (int i = 1; i < m; ++i) {
            for (int j = 1; j < n; j++) {
                f[i][j] = f[i - 1][j] + f[i][j - 1];
            }
        }
        return f[m - 1][n - 1];
    }
}

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

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func uniquePaths(m int, n int) int {
    f := make([][]int, m)
    for i := range f {
        f[i] = make([]int, n)
        for j := range f[i] {
            f[i][j] = 1
        }
    }
    for i := 1; i < m; i++ {
        for j := 1; j < n; j++ {
            f[i][j] = f[i-1][j] + f[i][j-1]
        }
    }
    return f[m-1][n-1]
}

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function uniquePaths(m: number, n: number): number {
    const f: number[][] = Array(m)
        .fill(0)
        .map(() => Array(n).fill(1));
    for (let i = 1; i < m; ++i) {
        for (let j = 1; j < n; ++j) {
            f[i][j] = f[i - 1][j] + f[i][j - 1];
        }
    }
    return f[m - 1][n - 1];
}

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/**
 * @param {number} m
 * @param {number} n
 * @return {number}
 */
var uniquePaths = function (m, n) {
    const f = Array(m)
        .fill(0)
        .map(() => Array(n).fill(1));
    for (let i = 1; i < m; ++i) {
        for (let j = 1; j < n; ++j) {
            f[i][j] = f[i - 1][j] + f[i][j - 1];
        }
    }
    return f[m - 1][n - 1];
};

Solution 3

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class Solution:
    def uniquePaths(self, m: int, n: int) -> int:
        f = [1] * n
        for _ in range(1, m):
            for j in range(1, n):
                f[j] += f[j - 1]
        return f[-1]

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class Solution {
    public int uniquePaths(int m, int n) {
        int[] f = new int[n];
        Arrays.fill(f, 1);
        for (int i = 1; i < m; ++i) {
            for (int j = 1; j < n; ++j) {
                f[j] += f[j - 1];
            }
        }
        return f[n - 1];
    }
}

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

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func uniquePaths(m int, n int) int {
    f := make([]int, n+1)
    for i := range f {
        f[i] = 1
    }
    for i := 1; i < m; i++ {
        for j := 1; j < n; j++ {
            f[j] += f[j-1]
        }
    }
    return f[n-1]
}

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function uniquePaths(m: number, n: number): number {
    const f: number[] = Array(n).fill(1);
    for (let i = 1; i < m; ++i) {
        for (let j = 1; j < n; ++j) {
            f[j] += f[j - 1];
        }
    }
    return f[n - 1];
}

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/**
 * @param {number} m
 * @param {number} n
 * @return {number}
 */
var uniquePaths = function (m, n) {
    const f = Array(n).fill(1);
    for (let i = 1; i < m; ++i) {
        for (let j = 1; j < n; ++j) {
            f[j] += f[j - 1];
        }
    }
    return f[n - 1];
};

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