A simple 1-pass algorithm for finding leaf nodes in a tree flattened to an array of dot-separated string keys. [C#-flavored pseudocode]

FindLeavesInFlatNodeArray.cs

// Precalculate levels according to separator characters.
var nodes = flattenedTree.Select(flatNode => new {
        Value = flatNode.Value,
        Level = flatNode.Value.Count(c => c == '.'),
        IsLeaf = false
    }).ToArray();

// Find leaves by comparing the levels of each two adjacent items.
for (int i = 1; i < nodes.Length; i++)
{
    if (nodes[i].Level == nodes[i - 1].Level)
    {
        nodes[i - 1].IsLeaf = true;
    }
    else if (nodes[i].Level == nodes[i - 1].Level + 1)
    {
        nodes[i - 1].IsLeaf = false;
    }
    else if (nodes[i].Level < nodes[i - 1].Level)
    {
        nodes[i - 1].IsLeaf = true;
        nodes[i].IsLeaf = false;
    }
}
nodes[nodes.Length - 1].IsLeaf = true;


/*

----- Random test #0 ------
0       False   611
1       False   611.328
2       False   611.328.980
3       False   611.328.980.232
4       True    611.328.980.232.225
4       False   611.328.980.232.498
5       False   611.328.980.232.498.152
6       True    611.328.980.232.498.152.300
6       True    611.328.980.232.498.152.486
4       True    611.328.980.232.992
3       True    611.328.980.649

----- Random test #1 ------
0       False   988
1       False   988.848
2       True    988.848.328
2       False   988.848.344
3       True    988.848.344.438
3       True    988.848.344.686

----- Random test #2 ------
0       True    506

----- Random test #3 ------
0       False   489
1       True    489.374

----- Random test #4 ------
0       False   650
1       True    650.333
1       True    650.701
1       False   650.946
2       True    650.946.505
2       False   650.946.510
3       False   650.946.510.827
4       True    650.946.510.827.646

----- Random test #5 ------
0       False   647
1       True    647.367
1       True    647.491

----- Random test #6 ------
0       False   921
1       False   921.635
2       True    921.635.182
2       True    921.635.898
2       True    921.635.902
1       True    921.845

----- Random test #7 ------
0       False   625
1       True    625.989

*/