目的：解决等价问题。
表示方法：森林，底层实现用数组
主要操作：find()、union()
优化手段：
①find()时路径压缩
②union()时按规模合并、按树高合并（依据情况而定）
Java实现：
class DisjointSet {int father[];DisjointSet(){}DisjointSet(int n) {this.father = new int[n];for (int i = 0; i < n; i++)father[i] = i;}int find(int x) {if (x != father[x])father[x] = find(father[x]); // 回溯、压缩路径return father[x];}void union(int x, int y) {x = find(x);y = find(y);if (x == y)return;if (x < y) {if (father[y] == y)father[x] = father[x] == x ? -2 : father[x] - 1;elsefather[x] += father[y];father[y] = x;} else {if (father[x] == x)father[y] = father[y] == y ? -2 : father[y] - 1;elsefather[y] += father[x];father[x] = y;}}
}
解决问题：迷宫生成问题、树节点的最近祖先、连通回路等
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