接雨水

int trap(vector<int>& height) {
        int n = height.size();
        // left[i]表示i左边的最大值，right[i]表示i右边的最大值
        vector<int> left(n), right(n);
        for (int i = 1; i < n; i++) {
            left[i] = max(left[i - 1], height[i - 1]);
        }
        for (int i = n - 2; i >= 0; i--) {
            right[i] = max(right[i + 1], height[i + 1]);
        }
        int water = 0;
        for (int i = 0; i < n; i++) {
            int level = min(left[i], right[i]);
            water += max(0, level - height[i]);
        }
        return water;
    }