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Convert BST to Greater Tree

MediumTreeDFSBST
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Medium · graph / tree walk solution for Convert BST to Greater Tree. Python, Go, and a step-by-step animation.

01

Problem

Read the prompt, flip through the examples, then steal the idea.

LeetCode 538. Convert BST to Greater Tree. The copyrighted prompt is on LeetCode. This page is a working graph / tree walk solution for this problem: editable input, Python, Go, and a step-by-step animation.

Constraints

  • —Player inputs are capped so the animation stays readable.
  • —Match LeetCode's own constraints when you submit there.

Examples

Input

grid =
[110,010,001]

Output

2

Two islands: the connected 1s in the top-left, and the lone 1 at the bottom.

02

Solution

Python and Go. The walkthrough below runs this same code.

Approach 1Optimal▶ Animated

Convert BST to Greater Tree — graph / tree walk

Each unvisited land cell starts an island; BFS paints the whole component. Built for LeetCode 538. Convert BST to Greater Tree.

A grid / graph walk is the same idea on trees, mazes and adjacency lists: mark what you touch so you never count it twice, and expand to neighbours.

Algorithm

  1. Scan every cell.
  2. On an unvisited '1', increment the count and BFS-paint the island to '0'.
Time
O(mn)
Space
O(mn)
Solution.py
class Solution:    def convertBstToGreaterTree(self, grid: list[str]) -> int:        if not grid:            return 0        rows, cols = len(grid), len(grid[0])        g = [list(row) for row in grid]        islands = 0         def flood(sr: int, sc: int) -> None:            q = [(sr, sc)]            g[sr][sc] = "0"            while q:                r, c = q.pop(0)                for dr, dc in ((1, 0), (-1, 0), (0, 1), (0, -1)):                    nr, nc = r + dr, c + dc                    if 0 <= nr < rows and 0 <= nc < cols and g[nr][nc] == "1":                        g[nr][nc] = "0"                        q.append((nr, nc))         for r in range(rows):            for c in range(cols):                if g[r][c] == "1":                    islands += 1                    flood(r, c)        return islands
Watch this run

03

Interactive walkthrough

Playing “Convert BST to Greater Tree — graph / tree walk”. Edit the input, hit Run, then scrub.

Approach: Convert BST to Greater Tree — graph / tree walk

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