项链 Chains

项链是一种变型数独。它的英文名称也指一种非常重要的数独技巧，但我们此处仅讨论变型数独。在这种题型中，数字不能在每行、每列、每个由链条串联形成的宫内重复。

Chains is a variant. This word can also points to an important technique, but we just talk about variant here. In Chains, numbers cannot repeat in each Row, Column or circles linked by chains.

本质上，它是一种锯齿数独，但它们之间有非常小的区别。例题中，A1所在的宫可以在项链数独中出现，但不能在锯齿数独中出现。换句话说，锯齿数独的每个宫都是正交相邻的单元格，但项链数独的宫可能通过对角线联通。因此，在项链数独中，我们更难聚焦于某个宫进行观察。

It is essentially Jigsaw, however there are slight difference between them. The Box with A1 in the example is allowed in Chains, but cannot appear in Jigsaw. In other words, Boxes in Jigsaw are connected horizontally or vertically, but Boxes in Chains may connected on diagonals. So, it's more difficult to focus a Box in Chains.