【leetcode】Candy（hard) 自己做出来了 但别人的更好

There are N children standing in a line. Each child is assigned a rating value.

You are giving candies to these children subjected to the following requirements:

• Each child must have at least one candy.
• Children with a higher rating get more candies than their neighbors.

What is the minimum candies you must give?

 

注意，像 6 5 5 4 这样的 可以分 2 1 2 1 就是数字相同的不需要糖果数相同

我的思路，分别用两个vector存储递增和递减所需要的糖果，最终结果取两个的最大值

如 权重    6 6 1 5 4 4 3 7 4 2 2 2 5 4 3 5

找递增     1 1 1 2 1 1 1 2 1 1 1 1 2 1 1 2   从前向后找，初始都是1，遇到递增的就加上他前面查找的数字

找递减     1 2 1 2 1 2 1 3 2 1 1 1 3 2 1 1   从后向前找，初始都是1，遇到递增的就加上他前面查找的数字

最终        1 2 1 2 1 2 1 3 2 1 1 1 3 2 1 2

int candy(vector<int> &ratings) {
        int ans = 0;
        vector<int> candysUp(ratings.size(), 1);
        vector<int> candysDown(ratings.size(), 1);
        for(int i = 1; i < ratings.size(); i++) //查找递增
        {
            if(ratings[i] > ratings[i - 1])
            {
                candysUp[i] += candysUp[i - 1];
            }
        }
        for(int i = ratings.size() - 2; i >= 0; i--) //查找递减
        {
            if(ratings[i + 1] < ratings[i])
            {
                candysDown[i] += candysDown[i + 1];
            }
        }

        for(int i = 0; i < ratings.size(); i++)
        {
            ans += max(candysUp[i], candysDown[i]);
        }
        return ans;
    }

 

大神只循环一遍的思路：

其实整体思想跟上面一样，但是上面需要循环很多次是因为递增和递减分别处理。因为递减时不知道最大的数应该加几。

大神的思路是递增的就像上面那样加，但是递减的，先都加1，如果后面仍是递减的，再把之前少加的给加回来（如前面有nSeqLen个数字少加了1，就把答案直接多加一个nSeqLen）。

    //大神循环一遍的代码
    int candy2(vector<int> &ratings) {
    int nCandyCnt = 0;///Total candies
    int nSeqLen = 0;  /// Continuous ratings descending sequence length
    int nPreCanCnt = 1; /// Previous child's candy count
    int nMaxCntInSeq = nPreCanCnt;
    if(ratings.begin() != ratings.end())
    {
        nCandyCnt++;//Counting the first child's candy.
        for(vector<int>::iterator i = ratings.begin()+1; i!= ratings.end(); i++)
        {
            // if r[k]>r[k+1]>r[k+2]...>r[k+n],r[k+n]<=r[k+n+1],
            // r[i] needs n-(i-k)+(Pre's) candies(k<i<k+n)
            // But if possible, we can allocate one candy to the child,
            // and with the sequence extends, add the child's candy by one
            // until the child's candy reaches that of the prev's.
            // Then increase the pre's candy as well.

            // if r[k] < r[k+1], r[k+1] needs one more candy than r[k]
            if(*i < *(i-1))
            {
                //Now we are in a sequence
                nSeqLen++;
                if(nMaxCntInSeq == nSeqLen)
                {
                    //The first child in the sequence has the same candy as the prev
                    //The prev should be included in the sequence.
                    nSeqLen++;
                }
                nCandyCnt+= nSeqLen;
                nPreCanCnt = 1;
            }
            else
            {
                if(*i > *(i-1))
                { 
                    nPreCanCnt++;
                }
                else
                {
                    nPreCanCnt = 1;
                }
                nCandyCnt += nPreCanCnt;
                nSeqLen = 0;
                nMaxCntInSeq = nPreCanCnt;
            }   
        }
    }
    return nCandyCnt;
}