代码随想录算法训练营第十四天 | 144. 二叉树的前序遍历 145. 二叉树的后序遍历 94. 二叉树的中序遍历

144. 二叉树的前序遍历

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var preorderTraversal = function(root) {
    let stack = [];
    let res = [];

    if (!root) return res;

    stack.push(root);
    while(stack.length) {
        let top = stack.pop();
        res.push(top.val);

        if (top.right) stack.push(top.right);
        if (top.left)  stack.push(top.left);
    }

    return res;
};

145. 二叉树的后序遍历

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var postorderTraversal = function(root) {
    let stack = [];
    let res = [];

    if (!root) return res;

    stack.push(root);
    while(stack.length) {
        let top = stack.pop();
        res.unshift(top.val);
        if (top.left) stack.push(top.left);
        if (top.right) stack.push(top.right);
    }

    return res;
};

94. 二叉树的中序遍历

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var inorderTraversal = function(root) {
    let stack = [];
    let res = [];

    let cur = root;
    
    while(cur || stack.length) {
        if (cur) {
            stack.push(cur);
            cur = cur.left;
        } else {
            cur = stack.pop();
            res.push(cur.val);
            cur = cur.right;
        }
    }

    return res;
};