84、美团算法题---根据先中序构建二叉树，并打印右视图

一、题目

 

二、实现

/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 * 求二叉树的右视图
 * @param xianxu int整型一维数组 先序遍历
 * @param zhongxu int整型一维数组 中序遍历
 * @return int整型一维数组
 */
function solve( xianxu ,  zhongxu ) {
    //1. 先构建二叉树
    let l1 = 0, h1 = xianxu.length-1;
    let l2 = 0, h2 = zhongxu.length-1;
    let root = build(xianxu, l1, h1, zhongxu, l2, h2);
    //2. 通过层次遍历，获取右视图
    let res = [];
    let q = [root];
    while(q.length){
        let len = q.length;
        for(let i=0;i<len;i++){
            let r = q.shift();
            if(r.left) q.push(r.left);
            if(r.right) q.push(r.right);
            if(i==len-1) res.push(r.val);
        }
    }
    return res;
}
function build(xianxu, l1, h1, zhongxu, l2, h2){
        if(l1>h1) return null;
        let root = new TreeNode(xianxu[l1]);
        let mid = 0; //左孩子节点个数
        mid = zhongxu.indexOf(root.val)-l2;
        root.left = build(xianxu,l1+1,l1+mid,zhongxu,l2,l2+mid-1);
        root.right = build(xianxu,l1+mid+1,h1,zhongxu,l2+mid+1,h2);
        return root;
}
function TreeNode(val){
    this.val = val;
    this.left = null;
    this.right = null;
}
module.exports = {
    solve : solve
};