solution.java

class Solution {
    public boolean isInterleave(String s1, String s2, String s3) {
        int n = s1.length(), m = s2.length();
        if (n + m != s3.length())
            return false;

        // Ensure s2 is the shorter string to reduce space
        if (m > n) {
            String tmp = s1;
            s1 = s2;
            s2 = tmp;
            int t = n;
            n = m;
            m = t;
        }

        boolean[] dp = new boolean[m + 1];
        dp[0] = true;

        // initialize using only s2
        for (int j = 1; j <= m; ++j) {
            dp[j] = dp[j - 1] && (s2.charAt(j - 1) == s3.charAt(j - 1));
        }

        for (int i = 1; i <= n; ++i) {
            dp[0] = dp[0] && (s1.charAt(i - 1) == s3.charAt(i - 1));
            for (int j = 1; j <= m; ++j) {
                boolean takeFromS1 = dp[j] && (s1.charAt(i - 1) == s3.charAt(i + j - 1));
                boolean takeFromS2 = dp[j - 1] && (s2.charAt(j - 1) == s3.charAt(i + j - 1));
                dp[j] = takeFromS1 || takeFromS2;
            }
        }

        return dp[m];
    }
}