Heights: [2,1,5,6,2,3]. Maintain increasing stack of indices. i=0, h=2: push 0. Stack: [0]
i=1, h=1: 1<2, pop 0. Area = 2×(1−(−1)−1)=2.
Push 1. Stack: [1]
i=2, h=5: push 2. Stack: [1,2]
i=3, h=6: push 3. Stack: [1,2,3]
i=4, h=2: pop 3, area = 6×1=6.
Pop 2, area = 5×2=10. Push 4. Stack: [1,4]
i=5, h=3: push 5. Stack: [1,4,5]
Pop remaining: areas 3, 8, 6. Maximum: 10.