Day63｜单调栈part02：84.柱状图中最大的矩形
class Solution {
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[()] 比较 ，()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while
int 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;
}
}