二叉树层次遍历(BFS)

框架

队列 unshift() push()

function levelTraverse(root) {
  let que = [];
  que.push(root);
  while (que.length !== 0) {
    let size = que.length;// 防止数组塌陷问题
    // 同一层
    for (let i =0 ; i < size; ++i) {
      let cur = que.shift();
      // do something with current node.....
      // 加入左孩子
      // 加入右孩子
    }
    
  }
}

实现二叉树的层次遍历

function TreeNode(val, left, right) {
    this.val = (val === undefined ? 0 : val)
    this.left = (left === undefined ? null : left)
    this.right = (right === undefined ? null : right)
}

function levelTraversal(root) {
    let que = [],
        result = [];
    que.push(root);
    while (que.length !== 0) {
        let level = [],
            size = que.length;
        // 一层中的节点
        for (let i = 0; i < size; ++i) {
            let cur = que.shift();
            // 同一层中的一个节点
            level.push(cur.val);
            if (cur.left !== null) {
                que.push(cur.left);
            }
            if (cur.right !== null) {
                que.push(cur.right);
            }
        }
        result.push(level);
    }
    return result;
}

测试

let root = {
    val: 1,
    left: {
        val: 2,
        left: {
            val: 4,
            left: null,
            right: null
        },
        right: {
            val: 5,
            left: null,
            right: null
        }
    },
    right: {
        val: 3,
        left: {
            val: 6,
            left: null,
            right: null
        },
        right: {
            val: 7,
            left: null,
            right: null
        }
    }
}
let result = levelTraversal(root);
console.log(result);
// [ [ 1 ], [ 2, 3 ], [ 4, 5, 6, 7 ] ]