学步园

Given s1, s2, s3, find whether s3 is formed by the interleaving of
s1 and s2.

For example,
Given:
s1 = "aabcc",
s2 = "dbbca",

When s3 = "aadbbcbcac", return true.
When s3 = "aadbbbaccc", return false.

思路：

其实是一个动态规划问题：设o1, o2, o3分别为在s1, s2, s3上的下标：

is_interleaving(o1, o2, o3) = is_interleaving(o1-1, o2, o3) || is_interleaving(o1, o2-1, o3);

题解：

class Solution {
public:
    bool is_interleaving(
        const string& s1, const string& s2, const string& s3,
        int o1, int o2, int o3)
    {
        if (o3 < 0)
            return true;
        else
            return (s1[o1]==s3[o3] ? is_interleaving(s1, s2, s3, o1 - 1, o2, o3 - 1) : false) ||
                (s2[o2]==s3[o3] ? is_interleaving(s1, s2, s3, o1, o2 - 1, o3 - 1) : false);
    }

    bool isInterleave(string s1, string s2, string s3) {
        if (s1.size() + s2.size() != s3.size())
            return false;
            
        return is_interleaving(s1, s2, s3, 
            s1.size() - 1, s2.size() - 1, s3.size() - 1);
    }
};