二叉树的递归

1.Minimum Depth of Binary Tree

    // 递归版，时间复杂度O(n)，空间复杂度O(logn)
	private static int minDepth(TreeNode root) {
		if (root == null)   return 0;  
		return Math.min(minDepth(root.left), minDepth(root.right))+1;
	}

2.Maximum Depth of Binary Tree

同上，递归版，// 时间复杂度O(n)，空间复杂度O(logn)
public int maxDepth(TreeNode root) {
    if (root == null) return 0;
    return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
}

3.Path Sum

return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22
// 时间复杂度O(n)，空间复杂度O(logn)
public boolean hasPathSum(TreeNode root, int sum) {
    if (root == null) return false;
    if (root.left == null && root.right == null) // leaf
        return sum == root.val;
    return hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val);
}

4.Path Sum II

        同上，返回路径  // 时间复杂度O(n)，空间复杂度O(logn)
	public List<List<Integer>> pathSum(TreeNode root, int sum) {
		List<List<Integer>> result = new ArrayList<>();
		ArrayList<Integer> cur = new ArrayList<>(); // 中间结果
		pathSum(root, sum, cur, result);
		return result;
	}

	private static void pathSum(TreeNode root, int gap, ArrayList<Integer> cur, List<List<Integer>> result) {
		if (root == null)  return;
		cur.add(root.val);
		if (root.left == null && root.right == null) {
			if (gap == root.val) {  result.add(new ArrayList<>(cur)); }
		}
		pathSum(root.left, gap - root.val, cur, result);
		pathSum(root.right, gap - root.val, cur, result);
		cur.remove(cur.size() - 1);   //添加的删除了
	}

5.Binary Tree Maximum Path Sum

    // 时间复杂度O(n)，空间复杂度O(logn)
    private int max_sum;
	private int dfs(TreeNode root) {
		if (root == null) return 0;

		int l = dfs(root.left);
		int r = dfs(root.right);

		int sum = root.val;
		if (l > 0) sum += l;
		if (r > 0) sum += r;

		max_sum = Math.max(sum, max_sum);
		return Math.max(l, r) > 0 ? Math.max(l, r) + root.val : root.val;
	}

 6、给一棵二叉树, 找到和为最小的子树, 返回其根节点。

public class Solution {
    private TreeNode subtree = null;
    private int subtreeSum = Integer.MAX_VALUE;

    public TreeNode findSubtree(TreeNode root) {
        helper(root);
        return subtree;
    }
    
    private int helper(TreeNode root) {
        if (root == null) {
            return 0;
        }
        
        int sum = helper(root.left) + helper(root.right) + root.val;
        if (sum <= subtreeSum) {
            subtreeSum = sum;
            subtree = root;
        }
        return sum;
    }
}

7、求从根（root）到叶（leaf）的所有路径

Input：{1,2,3,#,5}
Output：["1->2->5","1->3"]
Explanation：
1
/ \
2 3
\
5

public class Solution {

    public List<String> binaryTreePaths(TreeNode root) {
        List<String> paths = new ArrayList<>();
        if (root == null) {
            return paths;
        }
        
        List<String> leftPaths = binaryTreePaths(root.left);
        List<String> rightPaths = binaryTreePaths(root.right);
        for (String path : leftPaths) {
            paths.add(root.val + "->" + path);
        }
        for (String path : rightPaths) {
            paths.add(root.val + "->" + path);
        }
        
        // root is a leaf
        if (paths.size() == 0) {
            paths.add("" + root.val);
        }
        
        return paths;
    }
}

　　

8、给定一棵二叉树，找到两个节点的最近公共父节点(LCA)。

最近公共祖先是两个节点的公共的祖先节点且具有最大深度。

public class Solution {
    // 在root为根的二叉树中找A,B的LCA:
    // 如果找到了就返回这个LCA
    // 如果只碰到A，就返回A
    // 如果只碰到B，就返回B
    // 如果都没有，就返回null
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode node1, TreeNode node2) {
        if (root == null || root == node1 || root == node2) {
            return root;
        }
        
        // Divide
        TreeNode left = lowestCommonAncestor(root.left, node1, node2);
        TreeNode right = lowestCommonAncestor(root.right, node1, node2);
        
        // Conquer
        if (left != null && right != null) {
            return root;
        } 
        if (left != null) {
            return left;
        }
        if (right != null) {
            return right;
        }
        return null;
    }
}

参考：九章算法

https://www.jiuzhang.com/

https://www.jiuzhang.com/solutions/minimum-subtree/

https://www.jiuzhang.com/solutions/binary-tree-paths/