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三次扫描

纵向计算！

class Solution {
public:
    int trap(vector<int>& height) {
        int n = height.size();
        if(n == 0) return 0;
        vector<int> left_max(n), right_max(n);
        left_max[0] = height[0];
        for(int i = 1; i < n; i ++) left_max[i] = max(left_max[i - 1], height[i]);
        right_max[n - 1] = height[n - 1];
        for(int i = n - 2; i >= 0; i --) right_max[i] = max(right_max[i + 1], height[i]);
        int res = 0;
        for(int i = 0; i < n; i ++) res += min(left_max[i], right_max[i]) - height[i];
        return res;
    }
};

单调栈

横向计算！

class Solution {
public:
    stack<int> stk;
    int trap(vector<int>& height) {
        int res = 0;
        for(int i = 0; i < height.size(); i ++){
            while(stk.size() && height[stk.top()] <= height[i]){
                int last = stk.top(); stk.pop();
                if(stk.empty()) break;
                res += (min(height[stk.top()], height[i]) - height[last]) * (i - stk.top() - 1);
            }
            stk.push(i);
        }
        return res;
    }
};

双指针(最优)

纵向计算！

class Solution {
public:
    stack<int> stk;
    int trap(vector<int>& height) {
        int n = height.size();
        if(!n) return 0;
        int left_max = height[0], right_max = height[n - 1];
        int l = 0, r = n - 1, res = 0;
        while(l < r){
            if(left_max < right_max) {
                res += left_max - height[l ++];
                left_max = max(left_max, height[l]);
            }else {
                res += right_max - height[r --];
                right_max = max(right_max, height[r]);
            }
        }
        return res;
    }
};