LeetCode-in-Java

64. Minimum Path Sum

Medium

Given a m x n grid filled with non-negative numbers, find a path from top left to bottom right, which minimizes the sum of all numbers along its path.

Note: You can only move either down or right at any point in time.

Example 1:

Input: grid = [[1,3,1],[1,5,1],[4,2,1]]

Output: 7

Explanation: Because the path 1 → 3 → 1 → 1 → 1 minimizes the sum.

Example 2:

Input: grid = [[1,2,3],[4,5,6]]

Output: 12

Constraints:

• m == grid.length
• n == grid[i].length
• 1 <= m, n <= 200
• 0 <= grid[i][j] <= 100

To solve the “Minimum Path Sum” problem in Java with the Solution class, follow these steps:

1. Define a method minPathSum in the Solution class that takes a 2D grid of non-negative numbers as input and returns the minimum sum of all numbers along the path from the top-left corner to the bottom-right corner of the grid.
2. Initialize a 2D array dp of size m x n, where dp[i][j] represents the minimum sum of the path from the top-left corner to position (i, j) in the grid.
3. Initialize dp[0][0] to the value of the top-left cell in the grid.
4. Initialize the first row and first column of dp based on the grid values and the previous cells in the same row or column.
5. Iterate over each position (i, j) in the grid, starting from the second row and second column:
   • Update dp[i][j] by adding the current grid value at (i, j) to the minimum of the values of the previous cells (i-1, j) and (i, j-1) in dp.
6. Return dp[m-1][n-1], which represents the minimum path sum from the top-left corner to the bottom-right corner of the grid.

Here’s the implementation of the minPathSum method in Java:

class Solution {
    public int minPathSum(int[][] grid) {
        int m = grid.length;
        int n = grid[0].length;
        int[][] dp = new int[m][n];
        
        dp[0][0] = grid[0][0];
        // Initialize first row
        for (int j = 1; j < n; j++) {
            dp[0][j] = dp[0][j-1] + grid[0][j];
        }
        // Initialize first column
        for (int i = 1; i < m; i++) {
            dp[i][0] = dp[i-1][0] + grid[i][0];
        }
        // Fill in the rest of the dp array
        for (int i = 1; i < m; i++) {
            for (int j = 1; j < n; j++) {
                dp[i][j] = grid[i][j] + Math.min(dp[i-1][j], dp[i][j-1]);
            }
        }
        return dp[m-1][n-1];
    }
}

This implementation efficiently calculates the minimum path sum using dynamic programming, with a time complexity of O(m * n) and a space complexity of O(m * n).

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