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The core idea
Define dp[i][j] = LCS length for first i chars of text1 and first j chars of text2.
If text1[i-1] == text2[j-1]: characters match, extend by 1.
dp[i][j] = dp[i-1][j-1] + 1.
Otherwise: take the best we can get without one of the characters.
dp[i][j] = max(dp[i-1][j], dp[i][j-1]).
Base: dp[0][j] = dp[i][0] = 0. Answer: dp[m][n].
