Leetcode 312. Burst Balloons

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Given n balloons, indexed from 0 to n-1. Each balloon is painted with a number on it represented by array nums. You are asked to burst all the balloons. If the you burst balloon i you will get nums[left] * nums[i] * nums[right] coins. Here left and right are adjacent indices of i. After the burst, the left and right then becomes adjacent.

Find the maximum coins you can collect by bursting the balloons wisely.

Note: 
(1) You may imagine nums[-1] = nums[n] = 1. They are not real therefore you can not burst them.
(2) 0 ≤ n ≤ 500, 0 ≤ nums[i] ≤ 100

Example:

Given [3, 1, 5, 8]

Return 167

    nums = [3,1,5,8] --> [3,5,8] -->   [3,8]   -->  [8]  --> []
   coins =  3*1*5      +  3*5*8    +  1*3*8      + 1*8*1   = 167

解析：典型的DP问题，从dp[1][1]迭代推出dp[1][n],n表示数组最后一个数的下标，递推关系为：dp[i][j] = max(dp[i][k-1] + nums[i-1]*nums[k]*nums[j+1]+dp[k+1][j]),这里的k范围是【i,j】，这里比较难的一点是nums[i-1]*nums[k]*nums[j+1]这一项的含义，为什么是这个，如果我们burst到最后的一个气球的时候，那么最后一次可获得的分数为nums[-1]*nums[k]*nums[n],再假设两个气球的情况下(如果1是最优k值)，dp[1][2] = dp[1][1] + dp[2][2] + nums[1]*nums[2]*1，由此类推，可以总结出上面的迭代公式。（此处没有给出明确的证明过程，如果有大神能够给出完整推导，还望能指点迷津）

Code:

class Solution {
public:
    int maxCoins(vector<int>& nums) {
        for(int i=0; i<nums.size(); i++){
            if(nums[i] == 0){
                nums.erase(nums.begin()+i);
                --i;
            }
        }
        
        const int n = nums.size();
        if(n == 0) return 0;
        
        nums.insert(nums.begin(),1);
        nums.insert(nums.end(),1);
        int m = nums.size();
        
        vector<vector<int>> dp(m,vector<int>(m,0));
        for(int k=1; k <= n; k++){
            for(int s = 1; s+k-1 <= n; s++){
                int best = 0;
                for(int b=0; b<k; b++){
                    best = max(best,dp[s][s+b-1]+nums[s-1]*nums[s+b]*nums[s+k]+dp[s+b+1][s+k-1]);
                }
                dp[s][s+k-1] = best;
            }
        }
        return dp[1][n];
    }
};

Leetcode 312. Burst Balloons

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原文地址：http://www.cnblogs.com/RookieCoder/p/5118784.html