chore: add optimize note for lc-1380

Author: theforestvn88

solution/1300-1399/1380.Lucky Numbers in a Matrix/README_EN.md

@@ -73,6 +73,20 @@ After the traversal is finished, we return the answer array.
 
 The time complexity is $O(m \times n)$, and the space complexity is $O(m + n)$. Where $m$ and $n$ are the number of rows and columns in the matrix, respectively.
 
+#### Optimize
+
+We can return the lucky number immediately when we found it, since there's only one lucky nunber. 
+We can prove this as follows:
+
+Given the input matrix contains only distinct numbers, now let suppose $[i,j]$ is the lucky number, we assume that there's another lucky number $[u,v]$, then we have:
+
+- $m[u][v]$ is the min of the row $u => m[u][v] < m[u][j] < m[i][j]$
+- $m[u][v]$ is the max of the col $v => m[u][v] > m[i][v] > m[i][j]$
+
+Thus $m[u][v] < m[i][j]$ and $m[u][v] > m[i][j]$, which is a contradiction.
+Therefore $[u,v]$ either does not exist or $[u,v]$ is the same as $[i,j]$. Since the matrix contains only distinct numbers, there is only one lucky number.
+
+
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 #### Python3