Leetcode 199. 二叉树的右视图

199. 二叉树的右视图

层次遍历二叉树(BFS)

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     public $val = null;
 *     public $left = null;
 *     public $right = null;
 *     function __construct($val = 0, $left = null, $right = null) {
 *         $this->val = $val;
 *         $this->left = $left;
 *         $this->right = $right;
 *     }
 * }
 */
class Solution {

    /**
     * @param TreeNode $root
     * @return Integer[]
     */
    function rightSideView($root) {
        $ans = [];
        if ( $root == null ) {
            return $ans;
        }
    
        $queue = [];
        array_push($queue, $root);
        while ( count($queue) > 0 ) {
            $len = count($queue);
            for ($i = 0; $i < $len; $i++) {
                $node = array_shift($queue);
                if ( $i+1 == $len ) { //右视图看到的层级遍历的最后一个节点
                    array_push($ans, $node->val);
                } 
				
				// if ( $i == 0 ) { //左视图看到的都是层级遍历的第一个节点
				// 		array_push($ans, $node->val);
				// }
                
                if ( $node->left ) {
                    array_push($queue, $node->left);
                }
                
                if ( $node->right ) {
                    array_push($queue, $node->right);
                }
            }
        }
        
        return $ans;
    }
}

深度遍历二叉树(DFS)

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     public $val = null;
 *     public $left = null;
 *     public $right = null;
 *     function __construct($val = 0, $left = null, $right = null) {
 *         $this->val = $val;
 *         $this->left = $left;
 *         $this->right = $right;
 *     }
 * }
 */

class Solution {
    /**
     * @param TreeNode $root
     * @return Integer[]
     */
    function rightSideView($root) {
        $ans = [];
        if ( $root == null ) {
            return $ans;
        }

        $this->dfs($root, 0, $ans);

        return $ans;
    }

    function dfs($root, $depth, &$ans) {
        if ( $root == null ) {
            return;
        }
		// 访问顺序:节点->右子树->左子树
        if ($depth === count($ans) ) { //仅记录每层第一次加入的元素
            array_push( $ans, $root->val );
        }
        $this->dfs( $root->right, $depth+1, $ans );
        $this->dfs( $root->left, $depth+1, $ans );
    }
}

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