739. 每日温度

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class Solution {public int[] dailyTemperatures(int[] temperatures) {int[] res = new int[temperatures.length];Deque<Integer> stack = new LinkedList();for(int i =0;i<temperatures.length;i++){if(stack.isEmpty()){stack.addLast(i);continue;}int tempIndex = stack.getLast();if(temperatures[i]<temperatures[tempIndex]){stack.addLast(i);continue;}while(!stack.isEmpty()&&temperatures[(tempIndex = stack.getLast())]<temperatures[i]){res[tempIndex] = i-tempIndex;stack.removeLast();}stack.addLast(i);}return res;}
}

496. 下一个更大元素 I

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class Solution {public int[] nextGreaterElement(int[] nums1, int[] nums2) {int[] arr = new int[nums1.length];Arrays.fill(arr,-1);Deque<Integer> stack = new LinkedList();for(int i =0;i<nums2.length;i++){if(stack.isEmpty()){stack.addLast(i);continue;}int tempIndex = stack.getLast();if(nums2[i]<nums2[tempIndex]){stack.addLast(i);continue;}while(!stack.isEmpty()&&nums2[(tempIndex = stack.getLast())]<nums2[i]){for(int j=0;j<nums1.length;j++){if(nums1[j]==nums2[tempIndex]){arr[j] = nums2[i];}}stack.removeLast();}stack.addLast(i);}return arr;}
}

503. 下一个更大元素 II

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代码

class Solution {public int[] nextGreaterElements(int[] nums) {int[] result = new int[nums.length];Arrays.fill(result,-1);Deque<Integer> stack = new LinkedList();for(int i =0;i<nums.length*2;i++){int index = i%nums.length;if(stack.isEmpty()){stack.addLast(index);continue;}int tempIndex = stack.getLast();if(nums[index]<nums[tempIndex]){stack.addLast(index);continue;}while(!stack.isEmpty()&&nums[(tempIndex = stack.getLast())]<nums[index]){result[tempIndex] = result[tempIndex]==-1?nums[index]:result[tempIndex];stack.removeLast();}stack.addLast(index);}return result;}
}

42. 接雨水

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代码

class Solution {public int trap(int[] height) {int size = height.length;if (size <= 2) return 0;// in the stack, we push the index of array// using height[] to access the real heightStack<Integer> stack = new Stack<Integer>();stack.push(0);int sum = 0;for (int index = 1; index < size; index++){int stackTop = stack.peek();if (height[index] < height[stackTop]){stack.push(index);}else if (height[index] == height[stackTop]){// 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indexstack.pop();stack.push(index);}else{//pop up all lower valueint heightAtIdx = height[index];while (!stack.isEmpty() && (heightAtIdx > height[stackTop])){int mid = stack.pop();if (!stack.isEmpty()){int left = stack.peek();int h = Math.min(height[left], height[index]) - height[mid];int w = index - left - 1;int hold = h * w;if (hold > 0) sum += hold;stackTop = stack.peek();}}stack.push(index);}}return sum;}
}

84. 柱状图中最大的矩形

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代码

class Solution {public int largestRectangleArea(int[] heights) {Stack<Integer> st = new Stack<Integer>();// 数组扩容，在头和尾各加入一个元素int [] newHeights = new int[heights.length + 2];newHeights[0] = 0;newHeights[newHeights.length - 1] = 0;for (int index = 0; index < heights.length; index++){newHeights[index + 1] = heights[index];}heights = newHeights;st.push(0);int result = 0;// 第一个元素已经入栈，从下标1开始for (int i = 1; i < heights.length; i++) {// 注意heights[i] 是和heights[st.top()] 比较 ，st.top()是下标if (heights[i] > heights[st.peek()]) {st.push(i);} else if (heights[i] == heights[st.peek()]) {st.pop(); // 这个可以加，可以不加，效果一样，思路不同st.push(i);} else {while (heights[i] < heights[st.peek()]) { // 注意是whileint mid = st.peek();st.pop();int left = st.peek();int right = i;int w = right - left - 1;int h = heights[mid];result = Math.max(result, w * h);}st.push(i);}}return result;}
}