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balloonio/leetcode_notes.md

Last active May 11, 2020 01:28
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Some notes for leetcode
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leetcode_notes.md

Palindrome

  • dynamic programing with f[i][j] = whether s[i:j+1] aka s[i]..s[j] is palindrome or its length
  • enumerate pivots, and expand from each pivot

Subsequence

  • if using dynamic programing, in general f[i][j] = f[i+1][j-1] || f[i][j-1] || f[i+1]f[j] (meaning you can either take both end, left only, or right only

DP

  • be careful with the initial state at the time of initialization
  • be careful to explicitly set the default state or negative state while iterating through f

2D Board Traversal

  • Two level nested loop; start a BFS each time when it's an island. Board level visited set. Boundary check on nx ny. 200

Binary Tree DFS

Four cases always to consider: 1) node == None 2) leaf node (this one is not necessary but safe to consider) 3) missing left branch 4) missing right branch

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