题目描述：

题目解答：

class Solution {
    public int trap(int[] height) {
        List<Integer> local_highest_index=new ArrayList<>();
	        if(height.length<=1){
	            return 0;
	        }
	        //第二步：找到局部最大点
	        for(int i=0;i<height.length;i++){
	            if(i==0){
	                if(height[i+1]<height[i])
	                    local_highest_index.add(i);
	            }else if(i!=height.length-1){
	                if(height[i]>height[i-1]&&height[i]>height[i+1]){
	                    local_highest_index.add(i);
	                }else if(height[i]>height[i-1]&&height[i]==height[i+1]){
	                    local_highest_index.add(i);
	                }
	                else if(height[i]==height[i-1]&&height[i]>height[i+1]){
	                    local_highest_index.add(i);
	                }
	            }else{
	                if(height[i]>height[i-1]){
	                    local_highest_index.add(i);
	                }
	            }
	        }
	        //第三步,寻找灌水的地方
	        ArrayList<int []> water_pair=new ArrayList<>();
	        for(int i=0;i<local_highest_index.size()-1;i++){
	            int index1=local_highest_index.get(i);
	            int index2=local_highest_index.get(i+1);
	            if(height[index2]>=height[index1]){
	                water_pair.add(new int[]{index1,index2});
	                continue;
	            }
	            for(int j=i+2;j<local_highest_index.size();j++){
	                if(height[local_highest_index.get(j)]>height[index2]&&height[index2]<height[index1]){
	                    index2=local_highest_index.get(j);
	                    i=j-1;
	                }                
	            }
	             water_pair.add(new int[]{index1,index2});
	        }
	         //第四步：灌水
	        int water=0;
	        for(int i=0;i<water_pair.size();i++){
	            int top=computeTop(height[water_pair.get(i)[0]],height[water_pair.get(i)[1]]);
	            for(int j=water_pair.get(i)[0];j<water_pair.get(i)[1];j++){
	                if(top>height[j]){
	                    water=water+top-height[j];
	                }
	            }
	        }
	        return water;
    }
    public int computeTop(int a,int b){
        if(a>=b){
            return b;
        }else{
            return a;
        }
    }
}

运行结果：

代码太长了，网络上有一种十分简洁的算法：

class Solution {
    public int trap(int[] A) {
        if (A==null) return 0;
        Stack<Integer> s = new Stack<Integer>();
        int i = 0, maxWater = 0, maxBotWater = 0;
        while (i < A.length){
            if (s.isEmpty() || A[i]<=A[s.peek()]){
                s.push(i++);
            }
            else {
                int bot = s.pop();
                maxBotWater = s.isEmpty()? // empty means no il
                0:(Math.min(A[s.peek()],A[i])-A[bot])*(i-s.peek()-1);
                maxWater += maxBotWater;
            }
        }
        return maxWater;
    }
}

运行效果如下；