leetcode 199 Solution

代码解析

package com.demo.s199;


import java.util.*;

/**
 * 给定一个二叉树的 根节点 root，想象自己站在它的右侧，按照从顶部到底部的顺序，返回从右侧所能看到的节点值。
 *
 *
 */

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;
    }
}

public class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> ret = new ArrayList<>();
        if(root == null) {
            return ret;
        }

        int max_dep = -1;
        //右分支视图按层输出
        Map<Integer,Integer> rightViewAtDepth = new HashMap<Integer, Integer>();
        //节点压栈
        Stack<TreeNode> nodeStack = new Stack<>();
        nodeStack.push(root);
        //深度压栈
        Stack<Integer> depthStack = new Stack<>();
        depthStack.push(0);
        while(nodeStack.size() != 0 ) {
            //节点弹栈
            TreeNode node = nodeStack.pop();
            //深度弹栈
            Integer dep = depthStack.pop();
            if(node != null) {
                //最大深度
                max_dep = Math.max(dep, max_dep);
                //右视图未遍历到该层，第一次遍历到的一定是右节点
                if(!rightViewAtDepth.containsKey(dep)) {
                    //存入视图
                    rightViewAtDepth.put(dep, node.val);
                }
                //左节点压栈
                nodeStack.push(node.left);
                //右节点压栈
                nodeStack.push(node.right);
                //层数存入深度栈
                depthStack.push(dep + 1);
                //层数存入深度栈
                depthStack.push(dep + 1);
            }
        }

        //遍历深度
        for(int i = 0 ;i <= max_dep; i++) {
            Integer val = rightViewAtDepth.get(i);
            ret.add(val);
        }
        return ret;
    }
}