[LeetCode]84 Largest Rectangle in Histogram

public class Solution {
public int largestRectangleArea(int[] height)
{
// Solution A
// return largestRectangleArea_Expand(height);

// Solution B
return largestRectangleArea_Stack(height);
}

////////////////////
// Solution A: Stack
//
private int largestRectangleArea_Stack(int[] h)
{
if (h == null || h.length == 0)
return 0;   // invalid input

Stack<Integer> stack = new Stack<Integer>();
int max = 0;
for (int i = 0 ; i < h.length ; i ++)
{
while (!stack.empty() && h[i] <= h[stack.peek()])
{
int last = stack.pop();
int area = 0;
if (stack.empty())
{
area = i * h[last];
}
else
{
// 从当前栈顶元素的下一个  stack.peek() + 1到
// 当前下标 i (exclusive)
// 都比出栈元素高度大
area = (i - (stack.peek() + 1)) * h[last];
}
max = Math.max(max, area);
}

stack.push(i);
}

while (!stack.empty())
{
int last = stack.pop();
int area = 0;
if (stack.empty())
{
area = h.length * h[last];
}
else
{
area = (h.length - stack.peek() - 1) * h[last];
}
max = Math.max(max, area);
}

return max;
}

////////////////////
// Solution B: Expand
//
// 对于每一个bar i, 寻找 最左边 >= h[i] 的 index
// 同理 找右边
// 计算最大
//
// O(n ^ 2)
private int largestRectangleArea_Expand(int[] h)
{
int max = 0;
for (int i = 0 ; i < h.length ; i ++)
{
// find left
int l = i;
while (l >= 0 && h[l] >= h[i])
l --;
l++;

// find right
int r = i;
while (r < h.length && h[r] >= h[i])
r ++;
r --;

max = Math.max(max, h[i] * (r - l + 1));
}
return max;
}
}