题目：

题解：

type pair struct {
    node *TreeNode
    left int
}

func pathSum(root *TreeNode, targetSum int) (ans [][]int) {
    if root == nil {
        return
    }

    parent := map[*TreeNode]*TreeNode{}

    getPath := func(node *TreeNode) (path []int) {
        for ; node != nil; node = parent[node] {
            path = append(path, node.Val)
        }
        for i, j := 0, len(path)-1; i < j; i++ {
            path[i], path[j] = path[j], path[i]
            j--
        }
        return
    }

    queue := []pair{{root, targetSum}}
    for len(queue) > 0 {
        p := queue[0]
        queue = queue[1:]
        node := p.node
        left := p.left - node.Val
        if node.Left == nil && node.Right == nil {
            if left == 0 {
                ans = append(ans, getPath(node))
            }
        } else {
            if node.Left != nil {
                parent[node.Left] = node
                queue = append(queue, pair{node.Left, left})
            }
            if node.Right != nil {
                parent[node.Right] = node
                queue = append(queue, pair{node.Right, left})
            }
        }
    }

    return
}