1. Word matched
Reaching the word's end means every letter matched along the path.
if (i === word.length) return trueTyporithm · medium
Word Search
▷ Interactive algorithm labword-searchbacktrackinggrid-dfsvisited-markerchoose-explore-undo
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Full lesson · 8 learning steps Open any exercise for focused practice
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Step 1 of 8 · intro card
Learn Word Search
Backtracking
DFS from each cell, matching letters and blocking the current cell, then restoring it on the way back.
Each starting cell launches a depth-first search that follows the word letter by letter.
Marking the current cell with a placeholder prevents the same cell being reused within one path.
Restoring the letter after exploring all four directions is the undo step, freeing the cell for other paths.
Preparing algorithm lab…
01
DFS in four directions.
02
Mark the cell, then restore it.
03
Prune on bounds and mismatch.
function exist(board, word) {
const rows = board.length;
const cols = board[0].length;
function backtrack(r, c, i) {
if (i === word.length) return true;
if (r < 0 || c < 0 || r >= rows || c >= cols) return false;
if (board[r][c] !== word[i]) return false;
const temp = board[r][c];
board[r][c] = "#";
const found =
backtrack(r + 1, c, i + 1) ||
backtrack(r - 1, c, i + 1) ||
backtrack(r, c + 1, i + 1) ||
backtrack(r, c - 1, i + 1);
board[r][c] = temp;
return found;
}
for (let r = 0; r < rows; r += 1) {
for (let c = 0; c < cols; c += 1) {
if (backtrack(r, c, 0)) return true;
}
}
return false;
}Read the implementation
Connect each part of the code to the concept before you type.
2. Out of bounds
Stop branches that leave the board.
if (r < 0 || c < 0 || r >= rows || c >= cols) return false3. Letter mismatch
Abandon a branch the moment the current letter does not match.
if (board[r][c] !== word[i]) return false4. Mark visited
Temporarily block this cell so the path cannot reuse it.
board[r][c] = "#"5. Restore (undo)
Put the letter back so other paths can use this cell.
board[r][c] = tempA complete learning loop
01
Understand
Read the concept map and follow the code breakdown.
02
Reconstruct
Type the implementation, repair mistakes and work through every recall exercise.
03
Verify
Use the final check without hints; revisit weak concepts in short practice sessions.