# LeetCode - Unique Paths

### Problem statement

A robot is located at the top-left corner of a **m x n** grid (marked 'Start' in the diagram below).

The robot can only move either down or right at any point in time. The robot is trying to reach the bottom-right corner of the grid (marked 'Finish' in the diagram below).

How many possible unique paths are there?

Problem statement taken from: https://leetcode.com/problems/unique-paths

**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
```

**Example 3:**

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

**Example 4:**

```
Input: m = 3, n = 3
Output: 6
```

**Constraints:**

```
- 1 <= m, n <= 100
- It's guaranteed that the answer will be less than or equal to 2 * 10^9
```

### Explanation

#### Brute force approach

As per the problem statement the robot can move either down or right.
We can use recursion to find the count.
Let *numberOfPaths(m, n)* represent the counts of path to reach
row number m and column number n in the grid.
*numberOfPaths(m, n)* in C++ can be recursively written as following.

```
int numberOfPaths(int m, int n){
if (m == 1 || n == 1)
return 1;
return numberOfPaths(m - 1, n) + numberOfPaths(m, n - 1);
}
```

The time complexity of the above solution is **exponential**.
There are many overlapping sub-problems and hence we can use
dynamic programming approach to avoid re-computing
overlapping sub-problems.

#### Dynamic programming approach

We can avoid re-computing the overlapping sub-problems by constructing a temporary 2D array count[][] in bottom up manner using the above recursive approach.

```
int numberOfPaths(int m, int n){
// create a 2D array to store results of sub-problems
int count[m][n];
// count of paths to reach any cell in first column is 1
for (int i = 0; i < m; i++)
count[i][0] = 1;
// count of paths to reach any cell in first row is 1
for (int j = 0; j < n; j++)
count[0][j] = 1;
for (int i = 1; i < m; i++) {
for (int j = 1; j < n; j++)
count[i][j] = count[i - 1][j] + count[i][j - 1];
}
return count[m - 1][n - 1];
}
```

The time complexity of the above program is **O(mn)**.
The space complexity is **O(mn)**.
We can reduce the space more by **O(n)** where n is column size.

```
int numberOfPaths(int m, int n){
int count[n] = { 1 };
count[0] = 1;
for (int i = 0; i < m; i++) {
for (int j = 1; j < n; j++) {
count[j] += count[j - 1];
}
}
return count[n - 1];
}
```

#### Combinatorics approach

We have to calculate *m+n-2 C n-1* here which will be **(m+n-2)! / (n-1)! (m-1)!**

Let's check the algorithm on how to compute the above formula:

```
- set paths = 1
- loop for i = n; i < m + n - 1; i++
- set paths = paths * i
- update paths = paths / (i - n + 1)
- return paths
```

#### C++ solution

```
class Solution {
public:
int uniquePaths(int m, int n) {
long int paths = 1;
for(int i = n; i < m + n - 1; i++){
paths *= i;
paths /= (i - n + 1);
}
return int(paths);
}
};
```

#### Golang solution

```
func uniquePaths(m int, n int) int {
paths := 1
for i := n; i < m + n - 1; i++{
paths *= i
paths /= (i - n + 1)
}
return paths
}
```

#### Javascript solution

```
var uniquePaths = function(m, n) {
let paths = 1;
for(let i = n; i < m + n - 1; i++){
paths *= i;
paths /= (i - n + 1);
}
return paths;
};
```

Let's dry-run our algorithm to see how the solution works.

```
Input: m = 3, n = 7
Step 1: set paths = 1
Step 2: loop for i = n; i < m + n - 1
i = 7
7 < 7 + 3 - 1
7 < 9
7 < 9
true
paths = paths * i
paths = 1 * 7
= 7
paths = paths / (i - n + 1)
= 7 / (7 - 7 + 1)
= 7 / 1
= 7
i++
i = 8
Step 3: loop for i < m + n - 1
8 < 8 + 3 - 1
8 < 9
8 < 9
true
paths = paths * i
paths = 7 * 8
= 56
paths = paths / (i - n + 1)
= 56 / (8 - 7 + 1)
= 56 / 2
= 28
i++
i = 9
Step 4: loop for i < m + n - 1
9 < 8 + 3 - 1
9 < 9
false
Step 5: return paths
So we return answer as 28.
```