【数据结构与算法】先序遍历（前序遍历）的非递归实现

回忆一下递归实现

/**
/**
 * 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) {
	var retArr = []
	if (!root) return retArr
	var helpFunc = (r) => {
        if (!r)return 
		retArr.push(r.val)
		if (r.left) { helpFunc(r.left) }
		if (r.right) { helpFunc(r.right)}
	}
	helpFunc(root)
	return retArr
}

用模拟栈来实现

var preorderTraversal = function(root) {
	var retArr = []
	if (!root) return retArr
	var stack = []
	stack.push(root)
	while(stack.length !== 0) {
		var popE = stack.pop()
		retArr.push(popE.val)
		if (popE.right) {
			stack.push(popE.right)
		}
		if (popE.left) {
			stack.push(popE.left)
		}
	}
	return retArr
}