力扣每日一题(+日常水几题)

121. 买卖股票的最佳时机 - 力扣（LeetCode）(很水)

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        int ans = 0;
        int pre = prices[0];
        for(auto & x : prices)
        {
            pre = min(pre,x);
            ans = max(ans, x - pre);
        }
        return ans;
    }
};

64. 最小路径和 - 力扣（LeetCode）

(很水) + 1 

class Solution {
public:
    int minPathSum(vectorint>>& grid) 
    {
        int m = grid.size();
        int n = grid[0].size();
        for(int i = 0; i < m; i++)
        {
            for(int j = 0; j < n; j++)
            {
                if(0 == i && 0 == j)continue;
                if(0 == i) grid[i][j] += grid[i][j - 1];
                else if(0 == j)grid[i][j] += grid[i - 1][j];
                else grid[i][j] += min(grid[i][j - 1],grid[i - 1][j]);
            }
        }         
        return grid[m - 1][n - 1];
    }
};

62. 不同路径 - 力扣（LeetCode）

和上一题想法一样,不过可以直接推答案

class Solution {
public:
    int uniquePaths(int m, int n) {
        using i64 = int64_t;
        i64 ans = 1;
        int x = n, y = 1;
        while(y < m)
        {
            ans = ans * x / y;
            x++;y++;
        }
        return ans;
    }
};

63. 不同路径 II - 力扣（LeetCode） 

 不额外开数组的方法

class Solution {
public:
    int uniquePathsWithObstacles(vectorint>>& obstacleGrid) 
    {
        int m = obstacleGrid.size();
        int n = obstacleGrid[0].size();
        for(int i = 0; i < m; i++)
        {
            for(int j = 0; j < n;j++)
            {
                obstacleGrid[i][j] ^= 1;
                if(!i && !j)continue;
                else if(!i) 
                {
                    if(obstacleGrid[i][j] == 0)continue;
                    obstacleGrid[i][j] += obstacleGrid[i][j - 1] - 1;
                }
                else if(!j)
                {
                    if(obstacleGrid[i][j] == 0)continue;
                    obstacleGrid[i][j] += obstacleGrid[i - 1][j] - 1;
                }
                else 
                {
                    if(obstacleGrid[i][j] == 0)continue;
                    obstacleGrid[i][j] += (obstacleGrid[i][j - 1] + obstacleGrid[i - 1][j]) - 1;
                }
            }
        }
        return obstacleGrid[m - 1][n - 1];
    }
};

    或者额外开一个数组

class Solution {
public:
    int uniquePathsWithObstacles(vectorint>>& obstacleGrid) 
    {
        int n = obstacleGrid.size(), m = obstacleGrid.at(0).size();
        vector <int> dp(m, 0);
        dp[0] = 1 - obstacleGrid[0][0];
        for (int i = 0; i < n; i++) 
        {
            for (int j = 0; j < m; j++) 
            {
                if(obstacleGrid[i][j] == 1)
                dp[j] = 0;
                else if(j && obstacleGrid[i][j - 1] == 0)//无需判断j == 0
                dp[j] += dp[j - 1];   
            }
        }
        return dp.back();
    }
};