#PHP, <s>100</s> <s>101</s> <s>99</s> 104 bytes

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	for($p=-1;$d=$argv[++$k];+$i<$p?:$i=$p,$x>$p?:$x=$p)$r[$p+=$k&1?$d:-$d]=$d;for(;$i<=$x;)echo$r[$i++]?:_;

takes input from command line arguments; run with `-nr`.

**breakdown**

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	for($p=-1;			// init position
		$d=$argv[++$k];	// loop $d through command line arguments
		+$i<$p?:$i=$p,			// 3. $i=minimum index
		$x>$p?:$x=$p			// 4. $x=maximum index
	)
		$r[
			$p+=$k&1?$d:-$d		// 1. jump: up for odd indexes, down else
		]=$d;					// 2. set result at that position to $d
	for(;$i<=$x;)			// loop $i to $x inclusive
		echo$r[$i++]?:_;		// print result at that index, underscore if empty