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前言

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二叉樹的遞歸分為「遍歷」和「分解問題」兩種思維模式，這道題需要用到「遍歷」的思維。

一、力扣971. 翻轉二叉樹以匹配先序遍歷

/*** 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 {List<Integer> res = new ArrayList<>();int[] voyageArr;int i = 0;boolean flag = true;public List<Integer> flipMatchVoyage(TreeNode root, int[] voyage) {voyageArr = voyage;fun(root);if(flag == false){List<Integer> list = new ArrayList<>();list.add(-1);return list;}return res;}public void fun(TreeNode root){if(root == null){return;}if(root.val != voyageArr[i++]){flag = false;return;}if(root.left != null && root.left.val != voyageArr[i]){res.add(root.val);TreeNode temp = root.left;root.left = root.right;root.right = temp;}fun(root.left);fun(root.right);}
}

二、力扣987. 二叉樹的垂序遍歷

/*** 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 {class Triple{TreeNode node;int row, col;public Triple(TreeNode node, int row, int col){this.node = node;this.row = row;this.col = col;}}LinkedList<List<Integer>> res = new LinkedList<>();List<Triple> nodes = new ArrayList<>();public List<List<Integer>> verticalTraversal(TreeNode root) {traverse(root,0,0);Collections.sort(nodes, (tri1,tri2)->{if(tri1.col != tri2.col){return tri1.col - tri2.col;}else if(tri1.row != tri2.row){return tri1.row - tri2.row;}else{return tri1.node.val - tri2.node.val;}});int pre = Integer.MIN_VALUE;for(int i = 0; i < nodes.size(); i ++){Triple cur = nodes.get(i);if(cur.col != pre){res.addLast(new LinkedList<Integer>());pre = cur.col;}res.getLast().add(cur.node.val);}return res;}public void traverse(TreeNode root, int row, int col){if(root == null){return ;}nodes.add(new Triple(root,row,col));traverse(root.left, row+1, col-1);traverse(root.right, row+1, col + 1);}
}

三、力扣666. 路徑總和 IV

class Solution {Map<Integer,Integer> tree = new HashMap<>();int sum = 0;public int pathSum(int[] nums) {for(int i = 0; i < nums.length; i ++){int value = nums[i]%10;int code = nums[i]/10;tree.put(code,value);}int rootCode = nums[0]/10;fun(rootCode,0);return sum;}public void fun(int code, int path){if(!tree.containsKey(code)){return;}int value = tree.get(code);int[] pos = decode(code);int depth = pos[0], id = pos[1];int leftCode = encode(depth+1, 2*id-1);int rightCode = encode(depth+1,2*id);if(!tree.containsKey(leftCode) && !tree.containsKey(rightCode)){sum += path + value;return;}fun(leftCode, path + value);fun(rightCode, path + value);}public int[] decode(int code){int id = code%10;int depth = code/10;return new int[]{depth,id};}public int encode(int depth, int id){return depth*10 + id;}
}