扁平数组转tree,tree转id组成的二维数组

case: [
					{ id: 1, level: 0, pid: 0 },
					{ id: 4, level: 1, pid: 1 },
					{ id: 5, level: 1, pid: 1 },
					{ id: 7, level: 2, pid: 5 },
					
					{ id: 2, level: 0, pid: 0 },
					{ id: 6, level: 1, pid: 2 },
					
					{ id: 3, level: 0, pid: 0 },
					{ id: 8, level: 1, pid: 3 },
					{ id: 9, level: 1, pid: 3 },
					{ id: 10, level: 2, pid: 9 },
				]


listToTree(list) {
			    var map = {}, node, tree= [], i;
			    for (i = 0; i < list.length; i ++) {
			        map[list[i].id] = list[i]; 
			        list[i].children = []; 
			    }
			    for (i = 0; i < list.length; i += 1) {
			        node = list[i];
			        if (node.pid !== 0) {
			            map[node.pid].children.push(node);
			        } else {
			            tree.push(node);
			        }
			    }
			    return tree;
			},
			treeToArray(list) {
			      const arr = []
			      let temp = []
			      const flatten = (list, arr) => {
			        list.forEach(item => {
			         if (item.children === null || item.children.length === 0) {
			           arr.push(temp.concat([item.id]))
			         } else {
			           temp.push(item.id)
			           flatten(item.children, arr)
			         }
			        })
			        temp = []
			        return arr
			      }
			      flatten(list, arr)
			      return arr
			    },