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  • [LeetCode] Triangle,

      

    Given a triangle, find the minimum path sum from top to bottom. Each step you may move to adjacent numbers on the row below.

    For example, given the following triangle

    [
         [2],
        [3,4],
       [6,5,7],
      [4,1,8,3]
    ]

     The minimum path sum from top to bottom is 11 (i.e., 2 + 3 + 5 + 1 = 11).

    Thoughts:

    It's a typical Dynamic Programming question, make sure be able to understand and solve it in mind,

    the formula should be: 

     dp[i][j] = dp[i+1][j] + dp[i+1][j+1]

    it is only correct and easier to solve it from bottom-up, the minimum value for theith value in a certain row equals to the value of the ith index plus the minimum value you can get between the ith and (i+1)th value from the lower row. since it is a Triangle, and the historical data from the lists can only be access once, we don't even need a two-dimensional array, then the formula becomes

    dp[j]=triangle.get(i).get(j) + Math.min(dp[j],dp[j+1]);

    So, the final code should be:

       public int minimumTotal(List<List<Integer>> triangle) {
         int length = triangle.size();
         if(length==0) return 0;
         if(length==1) return triangle.get(0).get(0);
        
         int[] dp = new int [length]; 
         
         //initialize with the values from the last row
         for(int i=0;i<triangle.get(length-1).size();i++){
             dp[i]=triangle.get(length-1).get(i);
          }
         
         //loop from the second last row
         for(int i=length-2;i>=0;i--){
             for(int j=0;j<triangle.get(i).size();j++)
                 dp[j]=triangle.get(i).get(j) + Math.min(dp[j],dp[j+1]);
         }
         return dp[0];
        }
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  • 原文地址:https://www.cnblogs.com/midan/p/4728877.html
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