力扣labuladong——一刷day61

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前言

二叉树的递归分为「遍历」和「分解问题」两种思维模式，这道题需要用到「分解问题」的思维，而且涉及处理子树，需要用后序遍历

一、力扣865. 具有所有最深节点的最小子树

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode subtreeWithAllDeepest(TreeNode root) {
        Result res = fun(root);
        return res.node;
    }
    public Result fun(TreeNode root){
        if(root == null){
            return new Result(null,0);
        }
        Result left = fun(root.left);
        Result right = fun(root.right);
        if(left.depth == right.depth){
            return new Result(root,left.depth+1);
        }
        Result res = left.depth > right.depth ? left : right;
        res.depth = res.depth + 1;
        return res;
    }
}
class Result{
    public TreeNode node;
    public int depth;
    public Result(TreeNode node, int depth){
        this.node = node;
        this.depth = depth;
    }
}

二、力扣1123. 最深叶节点的最近公共祖先

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode lcaDeepestLeaves(TreeNode root) {
        Result res = fun(root);
        return res.node;
    }
    public Result fun(TreeNode root){
        if(root == null){
            return new Result(null,0);
        }
        Result left = fun(root.left);
        Result right = fun(root.right);
        if(left.depth == right.depth){
            return new Result(root,left.depth+1);
        }
        Result res = left.depth > right.depth ? left : right;
        res.depth = res.depth + 1;
        return res;
    }
}
class Result{
    public TreeNode node;
    public int depth;
    public Result(TreeNode node, int depth){
        this.node = node;
        this.depth = depth;
    }
}

三、力扣1026. 节点与其祖先之间的最大差值

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int res = 0;
    public int maxAncestorDiff(TreeNode root) {
        fun(root);
        return res;
    }
    public int[] fun(TreeNode root){
        if(root == null){
            return new int[]{Integer.MAX_VALUE,Integer.MIN_VALUE};
        }
        int[] leftMinMax = fun(root.left);
        int[] rightMinMax = fun(root.right);
        int curMin = Math.min(Math.min(leftMinMax[0],rightMinMax[0]),root.val);
        int curMax = Math.max(Math.max(leftMinMax[1],rightMinMax[1]),root.val);
        res = Math.max(res,Math.max(curMax - root.val, root.val - curMin));
        return new int[]{curMin,curMax};
    }
}

四、力扣1120. 子树的最大平均值

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    double res = 0;
    public double maximumAverageSubtree(TreeNode root) {
        fun(root);
        return res;
    }
    public double[] fun(TreeNode root){
        if(root == null){
            return new double[]{0,0};
        }
        double[] left = fun(root.left);
        double[] right = fun(root.right);
        double curCount = left[0] + right[0] + 1;
        double curSum = left[1] + right[1] + root.val;
        res = Math.max(res,curSum/curCount);
        if(curCount == 1){
            return new double[]{curCount,root.val};
        }
        return new double[]{curCount,curSum};
    }
}