题目：给出三个字符串:s1、s2、s3，判断s3是否由s1和s2交叉构成。
class Solution {
public:
/**
* Determine whether s3 is formed by interleaving of s1 and s2.
* @param s1, s2, s3: As description.
* @return: true of false.
*/
bool isInterleave(string s1, string s2, string s3) {
// write your code here
if (s3.length() != s1.length()+s2.length())
return false;
if (s1.length() == 0)
return s2 == s3;
if (s2.length() == 0)
return s1 == s3;
vector<vector<bool> > dp(s1.length()+1, vector<bool>(s2.length()+1, false));
dp[0][0] = true;
for (int i = 1; i <= s1.length(); i++)
dp[i][0] = dp[i-1][0]&&(s3[i-1] == s1[i-1]);
for (int i = 1; i <= s2.length(); i++)
dp[0][i] = dp[0][i-1]&&(s3[i-1] == s2[i-1]);
for (int i = 1; i <= s1.length(); i++) {
for (int j = 1; j <= s2.length(); j++) {
int t = i+j;
if (s1[i-1] == s3[t-1])
dp[i][j] = dp[i][j]||dp[i-1][j];
if (s2[j-1] == s3[t-1])
dp[i][j] = dp[i][j]||dp[i][j-1];
}
}
return dp[s1.length()][s2.length()];
}
};