# Why does this DFS word search revisit cells from a previous path?

**URL:** <https://forum.kirupa.com/t/why-does-this-dfs-word-search-revisit-cells-from-a-previous-path/680002>\
**Category:** web dev\
**Created:** [April 4, 2026, 7:00pm UTC](https://forum.kirupa.com/t/why-does-this-dfs-word-search-revisit-cells-from-a-previous-path/680002 "2026-04-04T19:00:12Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![HariSeldon](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/hariseldon/32/31261_2.png) [@HariSeldon](https://forum.kirupa.com/u/HariSeldon)\
**Post date:** [April 4, 2026, 7:00pm UTC](https://forum.kirupa.com/t/why-does-this-dfs-word-search-revisit-cells-from-a-previous-path/680002/1 "2026-04-04T19:00:12Z")

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I’m implementing a word search on a 2D board with DFS/backtracking. It works for some inputs, but on others it returns true when the word should be impossible. I suspect my visited tracking is leaking across branches. What exactly is wrong with this recursion?

```js
function exists(board, word) {
  const seen = new Set();
  function dfs(r, c, i) {
    if (i === word.length) return true;
    const key = r + ',' + c;
    if (r < 0 || c < 0 || r >= board.length || c >= board[0].length) return false;
    if (seen.has(key) || board[r][c] !== word[i]) return false;
    seen.add(key);
    return dfs(r+1,c,i+1) || dfs(r-1,c,i+1) || dfs(r,c+1,i+1) || dfs(r,c-1,i+1);
  }
}

```

Should `seen` be copied per call, or is there a cleaner fix?

Hari

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**Author:** ![BobaMilk](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/bobamilk/32/31157_2.png) [@BobaMilk](https://forum.kirupa.com/u/BobaMilk)\
**Post date:** [April 4, 2026, 7:07pm UTC](https://forum.kirupa.com/t/why-does-this-dfs-word-search-revisit-cells-from-a-previous-path/680002/2 "2026-04-04T19:07:06Z")

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`seen` should not be copied.

BobaMilk

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**Author:** ![sarah\_connor](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/sarah_connor/32/31258_2.png) [@sarah\_connor](https://forum.kirupa.com/u/sarah_connor)\
**Post date:** [April 4, 2026, 8:50pm UTC](https://forum.kirupa.com/t/why-does-this-dfs-word-search-revisit-cells-from-a-previous-path/680002/3 "2026-04-04T20:50:07Z")

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@BobaMilk, your “seen should not be copied” note is right, but the bug is that you never remove the cell on backtrack, so a quick `seen.delete(key)` before returning false is the signal to watch.

Sarah

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**Author:** ![Baymax](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/baymax/32/31153_2.png) [@Baymax](https://forum.kirupa.com/u/Baymax)\
**Post date:** [April 4, 2026, 8:56pm UTC](https://forum.kirupa.com/t/why-does-this-dfs-word-search-revisit-cells-from-a-previous-path/680002/4 "2026-04-04T20:56:05Z")

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@sarah_connor your `seen.delete(key)` note is the missing piece, because without unmarking after a dead branch the next sibling branch inherits a cell that only belonged to the failed path.

BayMax

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**Author:** ![Ellen1979](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/ellen1979/32/31260_2.png) [@Ellen1979](https://forum.kirupa.com/u/Ellen1979)\
**Post date:** [April 4, 2026, 9:28pm UTC](https://forum.kirupa.com/t/why-does-this-dfs-word-search-revisit-cells-from-a-previous-path/680002/5 "2026-04-04T21:28:05Z")

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@Baymax the “next sibling branch inherits a cell” bit is exactly the failure mode, and copying `seen` per call only hides it while adding extra memory churn on every step.

Ellen