@@ -391,6 +391,7 @@ class Solution {
391391 }
392392}
393393```
394+ 394395动态规划法
395396``` java
396397class Solution {
@@ -418,6 +419,52 @@ class Solution {
418419 }
419420}
420421```
422+ 423+ 单调栈法
424+ ``` java
425+ public int trapMonoStack(int [] height){
426+ int size = height. length;
427+ 428+ if (size <= 2 ) return 0 ;
429+ 430+ // in the stack, we push the index of array
431+ // using height[] to access the real height
432+ Stack<Integer > stack = new Stack<Integer > ();
433+ stack. push(0 );
434+ 435+ int sum = 0 ;
436+ for (int index = 1 ; index < size; index++ ){
437+ int stackTop = stack. peek();
438+ if (height[index] < height[stackTop]){
439+ stack. push(index);
440+ }else if (height[index] == height[stackTop]){
441+ // 因为相等的相邻墙,左边一个是不可能存放雨水的,所以pop左边的index, push当前的index
442+ stack. pop();
443+ stack. push(index);
444+ }else {
445+ // pop up all lower value
446+ int heightAtIdx = height[index];
447+ while (! stack. isEmpty() && (heightAtIdx > height[stackTop])){
448+ int mid = stack. pop();
449+ 450+ if (! stack. isEmpty()){
451+ int left = stack. peek();
452+ 453+ int h = Math . min(height[left], height[index]) - height[mid];
454+ int w = index - left - 1 ;
455+ int hold = h * w;
456+ if (hold > 0 ) sum += hold;
457+ stackTop = stack. peek();
458+ }
459+ }
460+ stack. push(index);
461+ }
462+ }
463+ 464+ return sum;
465+ }
466+ ```
467+ 421468Python:
422469
423470双指针法
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