【LeetCode】142. 接雨水

方法一：动态规划

代码

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

        vector<int> rightMax(n);
        rightMax[n - 1] = height[n - 1];
        for (int i = n - 2; i >= 0; --i) {
            rightMax[i] = max(rightMax[i + 1], height[i]);
        }

        int ans = 0;
        for (int i = 0; i < n; ++i) {
            ans += min(leftMax[i], rightMax[i]) - height[i];
        }
        return ans;
    }
};

方法二：单调栈

代码

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

方法三：双指针

代码

class Solution {
public:
    int trap(vector<int>& height) {
        int n = height.size();
        int left = 0, right = n - 1;
        int leftMax = 0, rightMax = 0;
        int ans = 0;

        while (left < right) {
            if (height[left] <= height[right]) {
                if (height[left] > leftMax) {
                    leftMax = height[left];
                }
                else {
                    ans += leftMax - height[left];
                }
                left++;
            }
            else {
                if (height[right] > rightMax) {
                    rightMax = height[right];
                }
                else {
                    ans += rightMax - height[right];
                }
                right--;
            }
        }

        return ans;
    }
};