2385. Amount of Time for Binary Tree to Be Infected

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DFS + BFS

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class Solution {
    public int amountOfTime(TreeNode root, int start) {
        Map<Integer, List<Integer>> nodeToNodeListMap = new HashMap<>();
        dfs(nodeToNodeListMap, root);

        int minutes = -1;

        Set<Integer> visitedNodeSet = new HashSet<>();
        visitedNodeSet.add(start);
        Queue<Integer> queue = new LinkedList<>();
        queue.offer(start);
        while (!queue.isEmpty()) {
            for (int i = queue.size(); i > 0; i--) {
                int node = queue.poll();
                List<Integer> toList = nodeToNodeListMap.get(node);
                if (toList != null) {
                    for (int to : toList) {
                        if (visitedNodeSet.contains(to)) {
                            continue;
                        }
                        queue.offer(to);
                        visitedNodeSet.add(to);
                    }
                }
            }

            minutes++;
        }

        return minutes;
    }

    private void dfs(Map<Integer, List<Integer>> nodeToNodeListMap, TreeNode root) {
        if (root.left != null) {
            int from = root.val, to = root.left.val;
            nodeToNodeListMap.computeIfAbsent(from, key -> new ArrayList<>()).add(to);
            nodeToNodeListMap.computeIfAbsent(to, key -> new ArrayList<>()).add(from);
            dfs(nodeToNodeListMap, root.left);
        }
        if (root.right != null) {
            int from = root.val, to = root.right.val;
            nodeToNodeListMap.computeIfAbsent(from, key -> new ArrayList<>()).add(to);
            nodeToNodeListMap.computeIfAbsent(to, key -> new ArrayList<>()).add(from);
            dfs(nodeToNodeListMap, root.right);
        }
    }
}

DFS

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class Solution {
    private static class State {
        private Integer startDepth;
        private int minutes;
    }

    public int amountOfTime(TreeNode root, int start) {
        State state = new State();
        dfs(state, root, 0, start);
        return state.minutes;
    }

    private int dfs(State state, TreeNode root, int depth, int start) {
        if (root == null) {
            return 0;
        }
        if (root.val == start) {
            state.startDepth = depth;
        }

        int leftDepth = dfs(state, root.left, depth + 1, start);
        boolean startInLeftTree = state.startDepth != null;
        int rightDepth = dfs(state, root.right, depth + 1, start);
        boolean startInRightTree = !startInLeftTree && state.startDepth != null;

        if (root.val == start) {
            state.minutes = Math.max(state.minutes, Math.max(leftDepth, rightDepth));
        } else if (startInLeftTree) {
            state.minutes = Math.max(state.minutes, state.startDepth - depth + rightDepth);
        } else if (startInRightTree) {
            state.minutes = Math.max(state.minutes, state.startDepth - depth + leftDepth);
        }

        return Math.max(leftDepth, rightDepth) + 1;
    }
}

此解法需画图加深理解。

References

2385. Amount of Time for Binary Tree to Be Infected