dynamic programming - Knapsack Problem: Difference between the two loops

Question

Although these are two different questions, I wish to know the difference between these two loops. What is the difference between iterating over all weights for each item and iterating over all items for each weight?

for(int i=0; i < n; i++)  
    for(int j=W; j>=wt[i]; j--) 
        dp[j] = max(dp[j] , val[i] + dp[j-wt[i]]); 


for (int i=0; i<=W; i++)
      for (int j=0; j<n; j++)
         if (wt[j] <= i)
            dp[i] = max(dp[i], dp[i-wt[j]] + val[j]);

0-1 Knapsack Fractional Knapsack

Answer 1

等待大神答复