树结构（有id和pid字段）数组，生成多层嵌套的json对象

/*
 * 传入的数组有id和父节点pid字段，通过它们的关联构造成一棵或多棵树结构
 * @param nodes 集合
 * @param treeRootId 根节点的id
 */
function createTreeData(nodes, treeRootId) {
    var groups = {};
    // 按父节点将节点分组
    for (var i in nodes) {
        if (!groups[nodes[i].pid]) {
            groups[nodes[i].pid] = [];
        }
        groups[nodes[i].pid].push(nodes[i]);
        if (treeRootPid && treeRootPid === nodes[i].id) { // 发现传入的根节点id作为节点id时，将根节点设置为该节点的父节点
            treeRootPid = nodes[i].pid;
        }
    }

    var rootNodes = groups[treeRootPid];
    groups[treeRootPid] = null; // [SAFEGUARD]防止自为父节点或互为父节点（有环图结构）导致的死循环
    function traverseTreeNodeGroup(treeNodeGroup) {
        for (var i in treeNodeGroup) {
            var node = treeNodeGroup[i];
            if (groups[node.id]) {
                node.children = groups[node.id];
                groups[node.id] = null; // [SAFEGUARD]防止自为父节点或互为父节点（有环图结构）导致的死循环
                traverseTreeNodeGroup(node.children);
            }
        }
    }
    traverseTreeNodeGroup(rootNodes);
    return rootNodes;
}

// 测试数组
var nodes = [
    {id: '01', pid: '0'},
    {id: '02', pid: '0'},
    {id: '011', pid: '01'},
    {id: '012', pid: '01'},
    {id: '021', pid: '02'},
    {id: '022', pid: '02'},
    {id: '0221', pid: '022'},
    {id: '0222', pid: '022'}
];
// 调用
createTreeData(nodes, '0');
/* 返回如下结构
[
    {
        "id": "01",
        "pid": "0",
        "children": [
            {
                "id": "011",
                "pid": "01"
            },
            {
                "id": "012",
                "pid": "01"
            }
        ]
    },
    {
        "id": "02",
        "pid": "0",
        "children": [
            {
                "id": "021",
                "pid": "02"
            },
            {
                "id": "022",
                "pid": "02",
                "children": [
                    {
                        "id": "0221",
                        "pid": "022"
                    },
                    {
                        "id": "0222",
                        "pid": "022"
                    }
                ]
            }
        ]
    }
]
*/