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Interview Classic 150 Questions P64 Minimum Path Sum

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2025-12-13

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Dynamic Programming

Problem:

P64 Minimum Path Sum

https://leetcode.cn/problems/minimum-path-sum/description/?envType=study-plan-v2&envId=top-interview-150

Since you can only move right or down, the previous step to a point can only be from left or above. Set dp[i][j] to represent the minimum path length to reach grid[i][j].
We have:

dp[i][j]=min(dp[i−1][j],dp[i][j−1])+grid[i][j];dp[i][j] = min(dp[i - 1][j], dp[i][j - 1]) + grid[i][j]; 

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#include <vector>
using std::vector;

class Solution {
    public:
        int minPathSum(vector<vector<int> > &grid)
        {
                // dp[i][j] represents the minimum path length to reach grid[i][j]
                int i, j, m = grid.size(), n = grid[0].size();
                vector<vector<int> > dp(m, vector<int>(n, 0));

                dp[0][0] = grid[0][0];

                for (j = 1; j < n; j++) {
                        dp[0][j] = dp[0][j - 1] + grid[0][j];
                }
                for (i = 1; i < m; i++) {
                        dp[i][0] = dp[i - 1][0] + grid[i][0];
                }

                for (i = 1; i < m; i++) {
                        for (j = 1; j < n; j++) {
                                dp[i][j] = std::min(dp[i - 1][j], dp[i][j - 1]) + grid[i][j];
                        }
                }
                return dp[m - 1][n - 1];
        }
};

leetcode hot 100 rewrite

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#include <vector>
using std::vector;

class Solution {
    public:
        int minPathSum(vector<vector<int> > &grid)
        {
                int i, j, m = grid.size(), n = grid[0].size();
                // m == grid.length
                // n == grid[i].length
                // 1 <= m, n <= 200
                // 0 <= grid[i][j] <= 200

                // You can only move down or right one step at a time.
                vector<vector<int> > dp(m, vector<int>(n, 0));

                dp[0][0] = grid[0][0];
                for (i = 1; i < m; i++)
                        dp[i][0] = dp[i - 1][0] + grid[i][0];

                for (j = 1; j < n; j++)
                        dp[0][j] = dp[0][j - 1] + grid[0][j];

                for (i = 1; i < m; i++) {
                        for (j = 1; j < n; j++)
                                dp[i][j] = std::min(dp[i - 1][j], dp[i][j - 1]) + grid[i][j];
                }

                return dp[m - 1][n - 1];
        }
};