Given a string s1, we may represent it as a binary tree by partitioning it to two non-empty substrings recursively.
Below is one possible representation of s1 = "great"
:
great / \ gr eat / \ / \ g r e at / \ a t
To scramble the string, we may choose any non-leaf node and swap its two children.
For example, if we choose the node "gr"
and swap its two children, it produces a scrambled string "rgeat"
.
rgeat / \ rg eat / \ / \ r g e at / \ a t
We say that "rgeat"
is a scrambled string of "great"
.
Similarly, if we continue to swap the children of nodes "eat"
and "at"
, it produces a scrambled string "rgtae"
.
rgtae / \ rg tae / \ / \ r g ta e / \ t a
We say that "rgtae"
is a scrambled string of "great"
.
Given two strings s1 and s2 of the same length, determine if s2 is a scrambled string of s1.
Analsysi:
We observe that if s1[0..i] and s2[0...i] are scramble strings, then there must have a k (0<=k<i) so that
1. s1[0..k] and s2[0...k] are scarable && s1[k+1...i] and s2[k+1...i] are scramble.
2. OR s1[0...k] and s2[i-k...i] are scramble && s1[k...i] and s2[0...i+k] are scramble.
We then define the define the state d[k][i][j] as whether the string s1[i...i+k] and s2[j...j+k] are scramble. We then have the formula:
d[k][i][j] = true; if for any l that 1<=l<=k-1, we have: 1. d[l][i][j] && d[k-l][i+l][j+l] || 2. d[l][i][j+k-l] && d[k-l][i+l][j].
Otherweise d[k][i][j] = false;
Solution:
1 public class Solution { 2 public boolean isScramble(String s1, String s2) { 3 if (s1.length()!=s2.length()) return false; 4 int len = s1.length(); 5 if (len==0) return true; 6 7 boolean[][][] d = new boolean[len+1][len][len]; 8 for (int i=0;i<len;i++) 9 for (int j=0;j<len;j++) 10 if (s1.charAt(i)==s2.charAt(j)) 11 d[1][i][j]=true; 12 else d[1][i][j]=false; 13 14 for (int k=2;k<=len;k++) 15 for (int i=0;i<=len-k;i++) 16 for (int j=0;j<=len-k;j++) 17 for (int l=1;l<=(k-1);l++){ 18 d[k][i][j]=false; 19 if (d[l][i][j] && d[k-l][i+l][j+l]){ 20 d[k][i][j]=true; 21 break; 22 } else if (d[l][i][j+k-l] && d[k-l][i+l][j]){ 23 d[k][i][j]=true; 24 break; 25 } 26 } 27 28 return d[len][0][0]; 29 } 30 }