Lua广度优先：网格最短步数的队列解法

-- 错误：一条道走到底的乱撞，第一条路不保证最短
local function dfs(x, y, steps)
    if x == tx and y == ty then
        best = math.min(best, steps)
        return
    end
    dfs(x + 1, y, steps + 1)
    dfs(x, y + 1, steps + 1)
end

local function bfs(sx, sy, tx, ty)
    local queue = {{sx, sy}}
    local dist = {[sx .. "_" .. sy] = 0}
    local head = 1
    while head <= #queue do
        local cur = queue[head]
        head = head + 1
        if cur[1] == tx and cur[2] == ty then
            return dist[cur[1] .. "_" .. cur[2]]
        end
        for _, d in ipairs({{1,0},{-1,0},{0,1},{0,-1}}) do
            local nx, ny = cur[1] + d[1], cur[2] + d[2]
            local nk = nx .. "_" .. ny
            if dist[nk] == nil then
                dist[nk] = dist[cur[1] .. "_" .. cur[2]] + 1
                queue[#queue + 1] = {nx, ny}
            end
        end
    end
    return -1
end