学习目标：

接雨水问题

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题目内容：

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具体步骤：

class Solution {
public:
    /**
     * max water
     * @param arr int整型vector the array
     * @return long长整型
     */
    long long maxWater(vector<int>& arr) {
        // write code here
        if (arr.size() == 0)
            return 0;
        long res = 0;
        int left = 0;
        int right = arr.size() - 1;
        int maxL = 0;
        int maxR = 0;
        
        while (left < right) {
            maxL = max(maxL, arr[left]);
            maxR = max(maxR, arr[right]);
            if (maxR > maxL)
                res += maxL - arr[left++];
            else
                res += maxR - arr[right--];
        }
        return res;
    }
};
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