Lexicographically Smallest Equivalent String

Given strings A and B of the same length, we say A[i] and B[i] are equivalent characters. For example, if A = "abc" and B = "cde", then we have 'a' == 'c', 'b' == 'd', 'c' == 'e'.

Equivalent characters follow the usual rules of any equivalence relation:

Reflexivity: 'a' == 'a'
Symmetry: 'a' == 'b' implies 'b' == 'a'
Transitivity: 'a' == 'b' and 'b' == 'c' implies 'a' == 'c'
For example, given the equivalency information from A and B above, S = "eed", "acd", and "aab" are equivalent strings, and "aab" is the lexicographically smallest equivalent string of S.

Return the lexicographically smallest equivalent string of S by using the equivalency information from A and B.



Example 1:

Input: A = "parker", B = "morris", S = "parser"
Output: "makkek"
Explanation: Based on the equivalency information in A and B, we can group their characters as [m,p], [a,o], [k,r,s], [e,i]. The characters in each group are equivalent and sorted in lexicographical order. So the answer is "makkek".
Example 2:

Input: A = "hello", B = "world", S = "hold"
Output: "hdld"
Explanation:  Based on the equivalency information in A and B, we can group their characters as [h,w], [d,e,o], [l,r]. So only the second letter 'o' in S is changed to 'd', the answer is "hdld".
Example 3:

Input: A = "leetcode", B = "programs", S = "sourcecode"
Output: "aauaaaaada"
Explanation:  We group the equivalent characters in A and B as [a,o,e,r,s,c], [l,p], [g,t] and [d,m], thus all letters in S except 'u' and 'd' are transformed to 'a', the answer is "aauaaaaada".


Note:

String A, B and S consist of only lowercase English letters from 'a' - 'z'.
The lengths of string A, B and S are between 1 and 1000.
String A and B are of the same length.

分析

把A,B建图，dfs这里就是S里每个c.dfs图，得到最小的c。每个路径一个seen

class Solution:
    def smallestEquivalentString(self, A: str, B: str, S: str) -> str:
        g = collections.defaultdict(list)
        for a,b in zip(A,B):
            g[a].append(b)
            g[b].append(a)
        def dfs(c,mn,seen):
            if c in seen:
                return mn
            seen.add(c)
            res = mn
            for n in g[c]:
                if n not in seen:
                    res=min(res,dfs(n,min(mn,n),seen))
            return res
        return ''.join([dfs(c,c,set()) for c in S])

UnionFind

class Solution:
    def smallestEquivalentString(self, A: str, B: str, S: str) -> str:
        f = {}
        def find(x):
            f.setdefault(x,x)
            if f[x] != x:
                f[x] = find(f[x])
            return f[x]
        def union(x,y):
            fx = find(x)
            fy = find(y)
            if fx < fy:
                f[fy] = fx
            else:
                f[fx] = fy 
        for a,b in zip(A,B):
            union(a,b)

        res = ''
        for c in S:
            res += find(c)

        return res