[itint5]交替字符串

http://www.itint5.com/oj/#17

DP。注意曾经把赋值写成了==，结果出错半天。

bool isInterleaving(string &str1, string &str2, string &str3) {
    int n = str1.length();
    int m = str2.length();
    int k = str3.length();
    if (n + m != k) return false;
    vector<vector<bool> > dp(n+1);
    for (int i = 0; i < n+1; i++) {
        dp[i].resize(m+1);
    }
    dp[0][0] = true;
    for (int i = 0; i <= n; i++) {
        for (int j = 0; j <= m; j++) {
            if (i == 0 && j == 0) {
                dp[0][0] = true;
            } else if (i == 0) {
                dp[0][j] = dp[0][j-1] && str2[j-1] == str3[j-1];
            } else if (j == 0) {
                dp[i][0] = dp[i-1][0] && str1[i-1] == str3[i-1];
            } else {
                dp[i][j] = (dp[i-1][j] && str1[i-1] == str3[i+j-1])
                            || (dp[i][j-1] && str2[j-1] == str3[i+j-1]);
            }
        }
    }
    return dp[n][m];
}