Largest rectangle in a histogram: pops, widths and areas
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heights [2, 1, 5, 6, 2, 3]
  i=0 height=2:
      push index 0   stack(top first) = [0]
  i=1 height=1:
      pop bar of height 2: left edge index -1, right edge index 0, width 1, area 2   best = 2
      push index 1   stack(top first) = [1]
  i=2 height=5:
      push index 2   stack(top first) = [2, 1]
  i=3 height=6:
      push index 3   stack(top first) = [3, 2, 1]
  i=4 height=2:
      pop bar of height 6: left edge index 2, right edge index 3, width 1, area 6   best = 6
      pop bar of height 5: left edge index 1, right edge index 3, width 2, area 10   best = 10
      push index 4   stack(top first) = [4, 1]
  i=5 height=3:
      push index 5   stack(top first) = [5, 4, 1]
  i=6 (sentinel) height=0:
      pop bar of height 3: left edge index 4, right edge index 5, width 1, area 3   best = 10
      pop bar of height 2: left edge index 1, right edge index 5, width 4, area 8   best = 10
      pop bar of height 1: left edge index -1, right edge index 5, width 6, area 6   best = 10
      push index 6   stack(top first) = [6]
largest rectangle = 10