Define dp[i][j] = minimum edits to convert first i chars of word1 to first j chars of word2.
If word1[i-1] == word2[j-1]: characters match, no edit needed.
dp[i][j] = dp[i-1][j-1].
Otherwise, take the minimum of:
- Insert:
dp[i][j-1] + 1 - Delete:
dp[i-1][j] + 1 - Replace:
dp[i-1][j-1] + 1
Base: dp[i][0] = i (delete all), dp[0][j] = j (insert all).