用一个变量记录深度，凡是遇到更深层次的直接取值
var findBottomLeftValue = function(root) {
var res = 0
var depth = -1
var dfs = (node,d) => {
if(d > depth){
depth = d
res = node.val
}
node.left && dfs(node.left,d + 1)
node.right && dfs(node.right,d + 1)
}
dfs(root,0)
return res
};
