解析

class Solution {
public:
  int trap(vector<int> &height) {
    int res = 0;
    int n = height.size();

    vector<int> leftH(n,0);
    vector<int> rightH(n,0);

    for(int i=1;i<n-1;i++){
        leftH[i] =max(leftH[i-1],height[i-1]);
    }
    for(int i=n-2;i>0;i--){
        rightH[i] =max(rightH[i+1],height[i+1]);
    }
    for (int i = 1; i < n - 1; i++) {
      int temp =min(leftH[i],rightH[i]);
      if (temp > height[i]) {
        res += temp - height[i];
      }
    }

    return res;
  }
};

class Solution {
public:
  int trap(vector<int> &height) {
    int res = 0;
    int n = height.size();
    int leftMax = 0;
    int rightMax = 0;

    int left=0;
    int right =n-1;
    while (left<right) {
        if(height[left]<height[right]){
            if(height[left]>=leftMax){
                leftMax=height[left];
            }else{
                res+=leftMax-height[left];
            }
            left++;

        }else{
            if(height[right]>=rightMax){
                rightMax=height[right];
            }else{
                res+=rightMax-height[right];
            }
            right--;
        }
    }
    return res;
  }
};