【入门夯实】数组元素的原地去重：哈希表辅助的O(n)去重

local function dedupBad(t)
    local seen = {}
    local out = {}
    for _, v in ipairs(t) do
        if not seen[v] then
            seen[v] = true
            table.insert(out, v)
        end
    end
    return out
end

local function dedupInPlace(t)
    local seen = {}
    local write = 0
    for read = 1, #t do
        if not seen[t[read]] then
            seen[t[read]] = true
            write = write + 1
            t[write] = t[read]
        end
    end
    for i = #t, write + 1, -1 do
        t[i] = nil
    end
end

local arr = { 1, 2, 2, 3, 3, 3, 4 }
dedupInPlace(arr)
print(#arr)

local function dedupBy(t, keyFn)
    local seen = {}
    local write = 0
    for read = 1, #t do
        local k = keyFn(t[read])
        if not seen[k] then
            seen[k] = true
            write = write + 1
            t[write] = t[read]
        end
    end
    for i = #t, write + 1, -1 do
        t[i] = nil
    end
end